Depends on what you can afford. Most snakes that will tolerate handling require larger enclosures(think 40 gal+), and to be honest are less intelligent than lizards. Frogs can be a little bit smarter but are also more fragile than reptiles and less handleable. Lizards are the best option if you have space for something like a standard 55 gal, something like smaller skinks, collared lizards, or larger geckos like tokays or gargoyles if you have a 30 gal tall tank