What to do with a 20 gallon tank

So I am have this 20 gallon tank sitting in a closet. It has been waiting for something to live in it for a long time now. I believe it is time to do something about that.

Here is the deal. It needs to be kept where my son can't get his hands on it. It also needs to be relatively easy to move when the time comes. Without question, it will also need to host some sort of saltwater creature. Freshwater is for pussies and old ladies.

Ideally, the setup will have a shallow layer of sand on the bottom, and be filled with rock. The rock will most likely be a little on the spendy side, because it will all be live. I will mail order it from one of many fine internet retailers. I could go local, but even with shipping, I may not get as good a deal. The lighting is up in the air. Filtration will be done with a CPR bakpak I bought quite some time ago, and never used. I will probably stick a powerhead in the tank for circulation, and of course, the heater.

With all that out of the way, I have a few ideas as to what I would keep:

1. Reef setup: This would require a bit of an investment in lighting and additives, and really, it would not be easy to pick up and move when the time comes, so I doubt I will do this.

2. Invert only setup: Nothing but invertebrates. Shrimps, crabs, snails, 1 starfish, maybe even try to keep a sea apple alive. Could be interesting, but I know I will eventually want something with fins swimming around in the tank.

3. Goby tank: Yep, just gobies, with maybe a shrimp or two. I am really looking into this actually. I saw some goby setups and thought they kicked much ass.

4. Frogfish: Otherwise known as anglers. Get a single one, feed it damsels. Frogfish can be expensive though.

5. Regular community: Small tank for it. Possibly have a clown of course, since my son would want his own Nemo. Maybe a shrimp, a goby along the bottom, and something else, maybe a pseudochromis, like an orchid or neon.

Now, placement is going to be tricky, and I have 1 idea. Upstairs in the "extra" room on a desk. I wonder how well that would work. Actually, that would not work at all. The floor up there is not as even as it should be. Oh well, it was just a thought. I guess I can stick a beta in a bowl.