If this was your tank, what would you put in it?

I've posted this elsewhere, forgive me if you've seen it before...

I need some help! I had a pair of saddles that I lost to brook a little while ago, then after agonizing over the next inhabitant I brought home a dwarf lion that didn't make it (looked like it died of 'shipping stress', not a parasite or bacterial infection). So now the tank is sitting empty, and I have no idea what to do with it. My husband wasn't too keen on the lion once he discovered it was poisonous and ate live shrimp, so I don't think we'll get another. Other than that, anything goes folks! I'm open to all suggestions, thanks for your help .

The tank is a 21g (24"L x 14"W x 15"H), it's skimmerless and has a large canister filter (FilStar XP2). The current inhabitants are a few Hawaiian hermits, zebra turbos, zoas and mushrooms. It's a FOWLR, with 2 x 15W NO fluorescent lighting so nothing light-hungry (no anemones). Non reef-safe fish/inverts are welcome, if I lose the shroom frags it's no big deal.

Hmm... I've considered more LR, but my other tanks are pretty packed and I actually like having one that looks more open. The tank has a filter rated for up to 75g with loads of biomedia, so the only thing I really worry about is nitrates. Because the tank is so small I'm happy doing hefty water changes if they're needed. The saddles were pretty big and there weren't any nutrient issues with them, but the tank is still pretty new so who knows what might happen down the road.
The sand is only about 1-1.5" thick. It's Bahamas oolite, which I think will work for a DSB, so I could add a few more inches - I'll have to think about it though, that's a lot of water to lose in a tiny tank.
I do have a very pretty macro growing in my 5.5g that might work well in this tank. It's a caulerpa as far as I can tell, but it's incredibly tiny and feathery. The runners are about 1mm thick, and the leaves form little bunches that look like powder puffs; the overall effect when it covers a rock is sort of fuzzy. There's also some sargassum starting to grow out of the LR in this tank. I have a bunch in the 5.5g as well, it's an interesting plant and more than welcome to stay.

I find them appealing, but I'm not sure if pipefish would like this much current. The tank has about 25x turnover right now - 300gph from the filter, and 230gph from the PH. Especially along the back wall, there's quite a bit of movement.