What kinds of small items could I put in a gift basket for the groomsmen?

Bottled water. Chapstick. Q-tips. Beef jerky and granola bars. Nail clippers.

Edit: these are filler items to fill out a gift basket to help them get through the weekend. For actual gifts, a personalized engraved flask, cuff links to be worn during the ceremony, nice new ties to be worn during the ceremony, iPod, nice cigars, are all small gifts.

Personalized or engraved pens would be great. Small, useful, classy and fits into basket. I think this one comes with a case. very nice.

http://www.engravedgiftcreations.com/gc101.html

Or maybe a pocket watch? also small and very classy.

http://www.engravedgiftcreations.com/gc225.html

Good luck on your shopping! I'm glad to hear you're doing it for the groomsmen.

A bottle of their favorite cologne and basically anything that can be made to a "survival kit" for them of things they will actually use on the wedding day: Tissues (some men are sensitive), bottled water, some hard candies and/or a small snack (incase anyone starts feeling like they are going to pass out due to their sugar dropping or they forgot/didn't have time to eat something before arriving), a small comb, a extra pair of socks (incase anyone forgot theirs), a Shout stain removal stick (for those accidental messes), Advil/Tylenol (may never know when you'll need it), etc.

Are you trying to get off cheap? Most guys like watches and they are small, fit in a basket. Guys like iPods. A lot like video games. I doubt you will find things like smelly soaps or lotions. You may have to cough up some bucks.

At the risk of sounding rude, that's the groom's job. They're his friends, it's his job to get them something.