Not quite sure why yo think Dwarf Seahorses are difficult. I have seen them in tanks just like this or even smaller for a single pair. feeding is the only thing most people have difficulties with since they require live food but hey it's just worth that for the coolness of the horses.
Here's a link if your interested in reading more that explains things pretty much the way I've seen them cared for.
Of course if your dead set against them then there's always SW shrimps/crabs that look pretty cool with maybe a single fire goby type fish.
What ever you choose though good luck! can't wait to see the pics!