I go to the botanical gardens all the time, with my Olympus E-500, camera bag, water bottle, and spare lens, and have never once had a problem. Heck, I even had my tripod with me for one photo shoot (I'd just gotten it and wanted to try it out), and no one said anything to me.
As for finding things to take pictures of, I think it depends on when you go, and what your 'pleasure' is ... I almost always come away with a ton of shots, but I do mostly nature & flower photography. During the summer, I always seem to come out of there with a ton of water lily photos ;)
This is probably one of my favorite photos I've taken there:
The whole set is here.