Peacocks and Gold Dusts are great and a pair would most likely thrive in a 40 gal tank but they are F-A-S-T. P. klemmeri (Neon Day geckos) are also a good choice along the same lines as Peacocks and Gold Dusts.

One of the small leaftail species could work as well. A trio of satanics would be great in a 40 gal. And they're not going to zip out every time you take the lid off the tank which is a bonus. High humidity and low temps and they'll do well.

Problem is that these species are usually available as wild caught rather than captive bred. You could house a crested gecko in a 40 gal without any problem and they are easier to maintain than any of the aforementioned species as well as quite a bit cheaper up front.

Pros and cons of everything so far:

Day geckos:
Pros: gorgeous, small, day active, fun to observe
Cons: Fast, fragile

Leaftails:
Pros: small, hesitant to flee, amazing camouflage
Cons: virtually inactive during daytime, very fragile, almost always wild caught

Cresteds:
Pros: hardy, captive bred, "laid back"
Cons: larger gecko=more food and more space

That's my 2 cents anyway. Good luck!
-----
lost in the jungle somewhere
Z