I used to bento a *lot*, both for us and for my clients. I don't do much currently, but there was nothing I wouldn't put into them. Some things work particularly well if you're going for artistry (grapes or edamame on skewers, cheese in shapes, colored rices, hard boiled eggs pressed into shapes, etc.); but, really, it's all about how creative you can get with a small space.
If you're really bored, you can see some of my bentos (most of these are old, I should probably have photographed some of the ones I was doing for my client:s before they moved) http://www.flickr.com/photos/falconinburgundy/tags/bento/ . I'd love to see some photos of other people's, too. There is, or once was, a livejournal community for bento that was pretty darn awesome. And, there's some spectacula stuff on flickr, too. There's one woman, who I used to have bookmarked, that does just crazy beautiful bentos, but I can't find her now. I'll link her for inspiration if I can find her again!
"This isn't life in the fast lane, it's life in oncomming traffic." -Terry Pratchett