Importance of nightly temp drop. I would say it's pretty important. It gives digestive systems a chance to rest a bit (not to mention the animal, itself).
Diurnal animals: For starters, you might consider a beardie. Alternatively, if you want something a little less common, a desert ig has a similar temperament. And, of course, if you don't mind the critters hiding quite a bit, you might get a kick out of croc skinks:
All three are omnivores; gut-loaded insects on a bed of fresh greens (to keep the insects healthy until they're munched) will keep them nice and happy. The first two, being desert species, require somewhat lower humidity than do the croc skinks. For any diurnal reptile, though, you need UV-B lights. Very different setups for them versus the leos, and definitely worth checking out the other forums specific to them for more (and better) info!
Personally, we have the croc skinks, and are planning on getting a pair of beardies this summer. Next year we might add desert igs, but, not yet... 
-----
----------====================----------
Justin, Julie and the menagerie...
www.baskingwonders.com
