A few ideas:

Leopard gecko
Fat-tailed gecko
Banded Gecko
House Gecko

Green Anole
Bahaman Anole
Whip-tailed lizard (AKA Vine-tailed lizard)
Orange-throated skink
Fence lizard
Sandfish skink (A personal favorite)


IF it is a 20 gallon standard or long:

Schneider's skink
Collared lizard
Fire skink
Rankin's dragon


IF it is a 20 gallon TALL:

Crested gecko
Day gecko (The small variety, not the giant)



When you decide on a sp., let us know so we can help you out with heating/lighting and whatnot.