Amazon Milk Frogs, and the various Polypedates (spelling?) species are often visible during the day, although as has been pointed out, tree frogs don't do much during the day time. All those beautiful photos of Red Eyes are really "lies" - the frogs are never that active during the day time, but fast asleep. The truly entertaining and active day time frogs are the dart frogs, particularly large species like Dendrobates tinctorius and Phyllobates terribilis. If you're looking for something diurnal that is a little less tricky for a beginner, try fire-bellied toads.