heya
why dont you try some newts? they arent really aggressive that I know of, but they can live in fairly shallow water, and you can take them out and hold them and stuff too, Ive seen some japanese newts in one of our LFS today, they look pretty cool.
you could probably catch some somewhere if you looked in the right places, shallow ponds etc,
or alternatively if you are looking for something a bit meatier and more aggressive seeming, why dont you get a big mama spider, like a tarantula or something.
I wanted one, but my boyfriend wont have a word of it..

haha so freshwater tropical fish it is!
Lemme know what u decide on.. theres so many things you can keep in a tank, I think even an ant colony would be interesting. LOL
keep us informed!
Starry^