Oh this is a tough question! I think the species that comes to mind most rapidly for fun amphibian to me is the Pumpkin Toadlet (Brachycephalus ephippium) - the frog species that is so small that their inner ear canals do not function enough for them to balance mid-jump, making them flop all over the place. That being said there are a BUNCH of other weird frog species with unique behaviors that I could probably mention here (but won't for the sake of brevity).

Also this is kinda unrelated but if you're really into Herpetology like I am and are looking for some fun reading material you should check out "Lost Frogs and Hot Snakes: Herpetologists' Tales from the Field" by Marty Crump. I did a book review on it for an Ecology course last semester and there's some really fascinating stuff in there :]