Hey Birds, welcome! You have some good space to offer, so you could keep multiple animals per tank. You may want to choose some medium to large amphibs, or you could fit a whole bunch of small ones.
White's tree frogs are a good size. You could fit a small colony of 5 or so in a 55 gallon, though they'd prefer it vertical. If you're up to keeping one large frog, african bullfrogs and marine toads both get pretty big. I'd only house one or 2 in a 55 gallon, though not one of each
Keep in mind, marine toads are fairly toxic though. Tiger salamanders are also fairly easy to keep. You could fit several in a tank that size. It all depends on what you're looking for.
but it all comes down to what you want and what interests you the most. But you could read up on tree frogs, they are one of the easier to care for groups.
