Well, I don’t have a “Man Cave”* per se* anymore, but the compromise I reached with my wife is that our media room (ceiling-mounted projector, with stadium seating using chairs from the Ciné Capri) has one of my pinball machines (Black Night 2000) and two video games (an upright Qbert, and a cocktail-table Missile Command). The decor is framed movie posters.
“Mad Scientist”
No, seriously - bits of electronics, computers, plastic widgets, scary machinery, tools… I tell ya, marrying a mad scientist/inventor is NOT as lucrative as those old movie would have you believe. No mansions or castles, no servant named Igor…
On the upside, we haven’t had peasants with pitchforks storming the place, either.
I’m a woman, and I just wanted to note for the record that I own a pinball machine, a pachinko machine, an external HD dedicated just to movie files, and about 2,000 books. I also own more than one bicycle and a complete set of bicycle tools. I used to own a 1961 Falcon in the 1990s. And I have a large collection of board games. Also, I built my own computer. It’s not just men who like big kid toys.
My mancave is decorated with trees and grass and dandelions and the occasional random cat wandering through. And those damn bottlebrush rats. If only the overfed cats would grab one of those along the way.
A man cave is where men keep their hobbies and the tools needed to keep the hobbies going. Often they contain certain creature comforts like a music system or a refrigerator. There will usually be multiple chairs to accommodate other hobbyists who may drop by. As time goes on it’s usually decorated with items related to the hobby.
It’s far more than just a place. It is the sanctum sanctorum. As Red Green notes: “It’s kind of a refuge, where he can be alone to do what he enjoys the most and what he does the best: * nothing*. No talking, no sharing, no moving, no thinking.”
Why aren’t there more females with female caves? Is RivkahChaya the only one? Is having a private cave so much of a male-only thang?
Children and men infesting the woman’s territory – namely the rest of the house other than the man cave.
and duct tape
Mine is basically a small office nook in our apartment. Right now there’s not much in it besides a PC workstation, some skis and golf clubs and other crap I need to go through.
I do want to add a leather chair and maybe a small TV, except we have enough TVs in the house.
Well, I can see a man cave having a theme and being a true cave if the guy is a decorator, but that’s the only case.
The father of one of my classmates was heavily into model trains. One of the rooms in their house was dedicated to his models; for Christmas, their Nativity would be part of the current setup.
I’m female but I live by myself, so the cave is either “the whole house” or “the table with the desktop computer on it”. Depending on what I’m doing it can be the table with the desktop computer and scanner, or the table with multiple computers, or the table with the drawing tablet and the computer. That big display coffee table has four drawers under the display one: they’re full of cables, external HDs and that kind of stuff.
I already asked that question here. It seemed to annoy some of the wimminfolk.
As for the OPs question, mine looks an awful lot like a boat.
Here in the UK, we call a man cave a ‘study’. 
My study has 3 computers in a local network, several large racks of CDs (alphabetised 
) a laser printer and a table to serve snacks / drinks on.
I never thought of my man cave as “themed”, but looking about there are beer glasses, beer signs, bongs, pipes, and music.
Beer, drugs, rock&roll?
Or college? 
Before I read** pullin**'s linked thread and get mad :D, I don’t have a cave because we just don’t have space. HE says it’s because the rest of the house is mine, which it really isn’t as it’s shared common areas. I guess by “mine” he means areas I insist are kept to a minimum cleanliness standard (not cluttered with crap and boxes everywhere). I don’t have my own space where I can put my desk and jewelry repair stuff, with a cozy reading spot and about a hundred plants. That sounds wonderful. I guess it’s the price I pay for keeping his crap contained in one room. 
I’m probably not alone.
I guess you’ve never met a woman heavily into quilting, ceramics, or other arts and crafts. That stuff can take up a ton of room and she often will need a dedicated space for them all.
You have a pinball machine plus you like computers and classic cars?
Where were you when I was getting married?
No seriously what do potential suitors think of all this awesome stuff?
Mine floats and is shaped like a sailboat hull.