With a tank that large I would do a killer paludarium
.
http://en.wikipedia.org/wiki/Paludarium
Something like an Amazon biotope or Swamp biotope. I am sure you could find some creatures that could enjoy/thrive in the conditions created. I think just the space allow would allow an amazing design.
Even if you only put 12" of water in it you could keep some interesting fish. Then add a small reptile like you want.
If you did a native swamp tank you could do a few sunfish, minnows, and maybe a crawfish or something in the water area. Then on the land area make some dry spots, some semi aquatic plants growing from the water area, a large tree or some limbs for climbing and it would easily house some Anoles. Possibly some geckos or a small snake of some kind too, if there are any that don't eat Anoles/Geckos. I don't know Reptiles that well. You sure do have lots of space to make something great.