If you live near the ocean, you could look about some rocky areas. I remember once, I found two ghost shrimp sized bright red shrimp in a little pool next to a large rock. Also, Ive found baby puffer fish ranging from a little under the size of a dime to quarter size in and near floating dead sea grass and other debres near boat docks. It would need to be let go if it starts to get big, but easy enough to replace, and they are pretty.
Im not sure what would stay small, but even taking some of the floating stuff that appears to be alive still and planting it in the tank, and running a net through some grass beds would get you something cool looking that could be kept for a time. Free to
Dont take any rocks however, unless it is legal in your area. I know its illegal in Florida, but I dont know if its everywhere.
If you dont live by the ocean, I dont know anything much about salt water fish, but Id guess shrimp with a bit of live rock.