Weird Al attending an A.V.A. function just does not compute with me. 
When I lived in Japan there was a cosplay convention at the same venue our convention was at. They only overlapped one day, our final one and their first.
Wadfest, the fan-made Discworld convention. I went to the first and third ones, in a field outside Derby. Dwarf-tossing, penguin-batting, a lot of drinking and cosplay by old men and women.
Oh. and then there was the time we built luggages and raced them.
I’ve been to many tattoo conventions. Pittsburgh’s Meeting of the Marked is having its 25th anniversary this year, and I’ve been to many of them along the way. I’m always a bit reluctant to use the men’s room; more than once I’ve had guys prepping for competition ask me to shave their backs, and there’s no polite way to say no. Worst part is oiling the tatt down after shaving.
I attended several PA state sheriff conventions and one national sheriff convention. I don’t know where else you could find that many bloated egos all crammed into a single room.
Basically a combination there-of and more. Pittsburgh runs the gamut from folks who treat it as a form of cosplay to some serious fetishists. They aren’t a bad group of folks as long as you don’t go into it with the attitude of “watching the freak show”; if you do that they can often spot you and single you out for some fun to be made. But the same can be said of almost any kind of convention one time or another. Even zombie things attract some of that; if you don’t believe me look up that great (?) film offering “Porn of the Dead” or something like “Stacy: Attack of the Schoolgirl Zombies”.
Attend either political party’s state/national conventions. 
In the early or mid 1990s, two groups were sharing a hotel for conventions.
The American Bible Society and the Hemp Industries Association. The mix was interesting, but “unusual.” I won’t say which group I belonged to.
I was at the Pokemon National Championships in Indianapolis when we noticed some bald kids running around. Then more bald kids…our first thought was cancer, but they looked perfectly healthy. Then we started seeing bald adults. It was a convention for people who had alopecia!
Next week I’ll be attending my fourth Ukulele World Congress. It’s held in a field outside Nashville, IN that is owned by a local ukulele purveyor and AFAIK not used for anything else through the year. Something like 800 people show up and mostly camp in the field.
The main event is an open mic on a large permanent stage that starts at 5PM on Friday and Saturday nights and runs until about 4AM. Everybody gets two songs. Some are well-known professionals, some are abject beginners, but either way you get two songs.
The best part is that the sort of people who will camp out in a field in Indiana and play ukuleles all weekend turn out to be really spectacular people.
I went to Chicago for a week for technical training, and the hotel also hosted the International Mr. Leather convention. Nothing I hadn’t seen in San Francisco, but the other two mid-westerners attending the training with me got themselves an education.
I work at a hotel. A couple of times a year, the Jehovah’s Witnesses hold a convention in my town. Fortunately, they come to socialize with each other, and they leave us heathens alone.
I would never join a church just to meet women, but, when they are in their 20s and early 30s, the JWs have some of the hottest babes on the planet.
One time, my town hosted the Family Motor Coach Association. These are people whose official address is a post office box somewhere in the middle of the country, but who live full-time in their RVs. The FMCA members stayed in their RVs. Every hotel in my city was packed to capacity with salesmen, trying to sell stuff to them. There are interior decorators and cabinetmakers who work full-time, year-round, doing custom work on motor coaches. We had one guy who sold TV satellite dishes. He made his sales quota for the entire year, in three days at the FMCA convention.
Maybe not that unusual, but I’ve been to lots of ornithological and birdwatching conventions. My favorite was the Neotropical Ornithology Convention in Cuzco, Peru. I’ve also been to conventions in Arizona, Ottawa, Costa Rica, South Africa, and Brazil.
I’ve been to a couple of conventions of IMATA, the International Marine Animal Trainers’ Association, back in the 1980’s – IIRC, one in Honolulu (where I lived at the time), and one in Long Beach. One trainer read a paper on her successful efforts to train a walrus. At the end of the talk, she took questions. Someone asked her if the walrus was ever belligerent, and if so how they handled that. Without a moment’s hesitation, she blurted out “We beat her senseless!”
About that time also, marine mammal researchers and academics were a subset of the American Society of Mammalogists. Then they created their own academic society, The Society for Marine Mammalogy, holding their charter convention in December (IIRC) 1981 in San Francisco. I was a charter member. (See their History page which includes a full list of Charter Members – Yes, my name is there.)
During the convention (which was 4 days long), the usual parade of researchers in the field (all of whom knew each other, of course) came up to read their papers. One of them suddenly stopped right in mid-sentence and stood there, totally blank, for a minute or so. Then the Moderator led him out a side door into the lobby, where (the word went around) he had a full-blown epileptic seizure.
I also went to a 4-day Orca convention held at University of Washington in Seattle, in 1980. I was a student at a community college in Hayward (Chabot College) in the S. F. Bay Area at the time, taking a freshman biology class. I had to cut all my classes for a week to attend, which I arranged ahead of time with all the instructors. The biology teacher asked me to keep an eye out for one Bemi DeBus, a whale conservation type of person with one of the major whale conservation groups of the day (the American Cetacean Society, IIRC). Once I got seated in the big auditorium on the first day, I discovered that she was sitting right next to me.
This was just a few weeks after the big volcano eruption at Mt. St. Helens. On the way up to Washington from the Bay Area, our plane flew right over it. The pilot dipped one wing so we could all get a better view, right down into the crater. It was still smoking. The surrounding area was solid gray for some large radius all around, and all the trees were laying on the ground like big piles of Pick-Up Stix.
In wallet making?
Not unusual of themselves but I was at the Libertarian Party’s national convention in Seattle and the National Association of Destroyer Veterans’ ditto in the same hotel at the same time. Some sidelong glances and visiting of each others’ public displays ensued, but no overt hostility. I asked one of the desk clerks if there’d been any incidents reported and he said, “One or two but not nearly like last month when we booked a gay men’s conference and the NFL draft at the same time.”
We attended the Pittsburgh Mr Leather Competition once years ago; we may have been the only straight couple in attendance. A friend of ours was one of the contestants and a few days before all the other competitors except for him and another guy had dropped out over some kind of internal honk-off. Lose in a field of 10 is one thing; lose in a field of 2 and you are going to want some friends close to hug and cry with. And it wasn’t that far from our house. Lets just say we had never quite seen that side of our friend before ------ but we were glad to show him some love.
PS – he was the one who lost. He got a bad question in that part of the competition and the other guy got a great one; like an underhanded pitch anyone could put over the fence. The other guy (IIRC) took at least a place at the Nationals.
Went to a convention for work once. The convention itself was fairly straight forward, but the food and after hour “networking parties” were out of the norm. All the food was Vegan and the parties had themes. The Vegan food tasted good, but several people were a little upset that there wasn’t even a meat choice. The themed parties were a good idea, but when you expect people to wear a hawaiian shirt or some other costume, you should tell them ahead of time, rather than when they show up to the convention; especially since 98% of us had to take a plane to the convention.
I found out later that their normal party planner had gone on to greener pastures and one of the salespeople had volunteered to fill in. The sales person was a Vegan and was described as ‘quirky’. So he planned the whole thing on what he liked rather than thinking that other people may eat meat, or don’t normally pack costumes and hawaiian shirts for a 3 day convention.
Not much to really relate. I’ve only gone to Computer Science conferences and computer/network/etc. trade shows.
But the CS ones are in really esoteric areas. The staff of the hotels and banquet places would give us some weird looks. Total nerd-ville.
The big problem: we weren’t big drinkers. Hotels would give a discount on rooms rates and such thinking we would drink up like normal conference folks. Nope. There was one where the manager was visibly steamed and wandering around asking people why they weren’t drinking. So we could only use a site once since the next time around the rates would jump.
SF Fan cons the rank and file are moderate drinkers (the authors, OTOH…) but have a short-fall in other areas.
The hotel-liaison for one of the WorldCons in NYC told an anecdote about how one afternoon he’d shown up to settle some point after which the hotel manager said, “And now, Ms. Destiny* would like to have a word with you.”
It turned out that she “was like the shop steward for the local hookers” and had come armed with a price sheet.
Destiny: For this it’s this much, for that it’s this much and–
Liaison: I, uh, I don’t think we’ll be needing your services.
Destiny: … Why not?
Liaison: Well, we’re a science-fiction convention.
Destiny: <lip curl> I’ve heard about you! You bring your own!
*Made up name.