Rincie,
As Carolyn points out, it depends on the size of the Seahorse; I've never kept dwarfs, so I'll speak to larger sized species only. Anything they can get their tales around works; they love all the Macro Algae?s for hitching and hunting and there's no one better than the other. Finger Leathers, Colt Corals and other such soft corals also work very well and are easy to keep. I?ve never had any luck with sponges, however Subastorey has a very nicely decorated tank with many sponges. I have several Purple, Golden and Orange Spiny Gorgonians and they too are easy varieties to keep alive.
Kevin