What is wrong with good old American Express travelers checks?
Or better yet, just go back to 1798 and bring a couple of goats with you.
Better to pick a commodity that was worth more in 1998 than it is now.
Movies on VHS. Assuming there is no limit on baggage in this time machine, take a truckload of (obviously pre-1999) movies on VHS, then sell them when you get there.
Hmmmm… gold was selling for around $300/oz in 1998. Today it’s just south of $1200/oz. So you’d be getting an exchange rate of about 1:4
Gas was $1.15/gallon (2.73 today, or 2.4X more)
Loaf of bread $1.26 ($2.27 today, or 1.8 X more)
dozen eggs $0.88 ($1.37 today, or 1.55X more)
postage stamp $0.32 ($0.44 today or 1.3 X more)
If you go the gold route, the holiday will end up costing you quite a lot more than if you stay in 2010.
It might look fraudulent to the Amex people because vendors will be redeeming checks with numbers that won’t exist for over ten years. From their point of view, I’d appear to be passing bad checks.
Bascially, say I gave Amex $100 and they gave me a check #54321. I cash the check at a bank in 1998, the bank sends it to Amex to redeem it for the $100. Amex says “We don’t have $100 for check #54321. Our checks only go up to 23456. Who gave that to you?”
Then I get arrested and will have to call the Time Consulate. The bureaucracy just makes your head spin. They’d go back to 1978, talk to my dad and I’d be grounded for a month!
Get a reasonable supply of coins and bills from pre-1998, and a sports almanac, or research the results from the nearest horse/dog track. Assuming you don’t muck up the timeline, you should be able to make however much money you need gambling. Might also want to stuff whatever you do not spend into an interest bearing account to enjoy upon your return to the present…
If you’re gonna drive to 1998, you might want to do something about your ID. Chances are your driver’s license will not have been issued yet. BYOB.
All you’d need is 1 or 2 bills from the previous timelime and a good scanner and printer. High rag paper and plenty of ink cartridges and no one would be the wiser!
StG
Does this really matter? Who on Earth would bother checking the year on a bunch of coins? Yes, someone getting the coin down the line might go “Huh, that’s odd” but it won’t be traceable to you. Nobody will be checking on your rolls of coins when you hand 'em in.
Honestly, I can’t remember ever looking at the date on any coin I have in recent memory.
However, a ton of change probably won’t get the job done. I’d have to agree with bup - video games. Old, used games are generally cheap and would turn into a huge profit if you could unload them not too long after release.
Gold bullion isn’t unusual - that’s just one site I found but I’m sure you could get some that are unmarked. 
Brand new PC games. CDs will fit easily in your luggage. Highest selling titles of all time, all from 1998:
#4: StarCraft
#5: Half Life
#32: Baldur’s Gate
Hell, you could probably make money off Diablo (#26) and Mist II (#42), both from 1997.
How are you going to avoid your past self?
I guess a better question would be, could you give me a lift to 1994? There are a few things I think I need to address in that year. Besides, I had a good time then.
Note that quarters from after 1998 would have a conspicuously different design on the reverse.
I’m pretty sure I’ve got all my old licenses and luckily I look roughly the same. At least I look enough like my crappy DL photo.
Well, I know where I was working at the time, so I’ll just avoid that town completely. I can give you a ride to 1998, but I’m not going any further back, so you’ll have to take a bus or hitch a ride the rest of the way to '94.
Now that is an awesome idea. I only need to find enough bills and coins to make about $50 and then let it grow from there.
Well, if there are going to be TWO mes in 1998 or 94 I can take 1998. Because I can’t go to the same place I was in 1998 (won’t that destroy the universe or something?). But since I’m still me I can do some background things to influence past-me to avoid a few bad situations. Let me know when you’re picking me up. I need to get a shitload of quarters.
Get a $20 bill from 1950, go back to that year and deposit or invest it, then go forward to 1998 and withdraw your compounded proceeds.
If you want to swing by Canada, the toonie was released in 1996. They are somewhat more manageable than quarters! There are still plenty of coins from pre-1998 floating about and it should be rather easy to get a couple hundred of them (I have one in my wallet right now). Of course, the exchange rate back then sucked: $1 CDN was only worth about $0.68 USD.
You’d get hosed VERY badly. This site shows gold was trading at about $400/ounce in 1996, and it’s now about $1150/oz.
I’d make friends with someone that has a business that deals with lots of cash - maybe a bookie, or someone who has a party store. Offer to buy all his old bills at a small premium. The problem might be getting 15 year old bills that don’t look 15 years old, too.
You could try diamonds - they’re small, light, and value-dense, but you’d probably have the same exchange premium problem, not to mention finding someone to take them. Better research your target date/place, too, to be sure there wasn’t a diamond heist in the last few years, or you will probably get a free visit to your not-so-friendly neighborhood police station interrogation room. Maybe you could grow a mustache and a South African accent, and claim to be Herr Zeil-Fone from the De Beers Diamond Company?
Someone suggested taking lots 'n lots of quarters. Better check your guidebook first. I thought there was a weight limit. Damned time carriers are trying any and all extra fees to make a buck in this economy.
Take a bunch of Susan B Anthony dollars. That way you can annoy every shopkeeper along your trip! Or, you know, take them to a bank and exchange them for bills. Alternately, you could download an ISO of and burn a bunch of Windows 98 CDs and sell them on the… wait, no craigslist wasn’t invented yet. Hmmmm… no, software piracy might be too much work for a vacation.
Pick up a couple thousand Enron stock certificates for a couple of pennies, and then take 'em back with you.