Here are a couple of photos of what I have done with my 100 gallon H. erectus tank. Since having the seahorses, I have "grown" to LOVE macroalgae. When I go into the LFS, I always ask if they have any. I have C. racemosa (green grape), Botryocladia pseudodichotoma (red grape), C. prolifera, and C. mexicana.
Since I did not want any types of corals, gorgonians or sponges (I don't want to make the tank any dirtier than it can get with having to feed something else), I bought three pieces of "Living Color" coral. I have 2 yellow branched corals and 1 tall red sponge. Besides the macroalgae garden, the seahorses are really fond of the red sponge. Since I added the red grape macro and the faux corals, my seahorses have gone from a dark, black/grey color to a burnt orange/yellow. They especially color up when hitched to the red sponge.