It depends entirely on what your friend thinks of as interesting fish. Does he mean oddballs? Predators? Or just something with a bit of personality?
If he's thinking community fish, there's a few to choose from. Khuulie loaches look like snakes, he might like those. Sparkling gouramis have great characters and are very inquisitive feisty little fish. Black phantom tetras add interest with their courtship display. Peacock gobies are nice. Shrimps are always great fun to watch. Corydoras are always active, but I suppose he might find those too cute.
Breedon's suggestion of crabs might be just the thing, if he's not looking for a community setup.
And a 60 ltr would be ideal for a setup of shelldwelling cichlids. That should be interesting enough for anyone, they have great character.
Or, as also suggested, he could set up a small group of dwarf puffers (I am assuming that he can get mature filter media from you). He'll need to do his homework though.