There is a fine line between fact and opinion. Especially in this hobby.

"Hermits are a nuisance." Fact. "Hermits are beneficial cuc." Fact.

The OP wanted to know what to put in his tank. There are no lights, so the recommendation of corals of any kind is a moot point, same with snails, since no light=no algae.

If you're comfortable letting someone drop in a few mysids to keep the critters alive, some hermits or nassarius would be the best option. (Only if you don't mind having hermits.)

If you're going out of town, the best thing to add to the tank is probably nothing.

Even if they'll stay alive while you're gone... if you put something into the tank and then leave town, you miss out on a valuable learning opportunity to watch your tank go through the next stage of development. And if you're really into your tank like I am, the first living things to be put in are also a bonding opportunity. I wouldn't bring home a new puppy and then leave it with strangers for a while. Sure, they're just crabs. But they're your crabs.