Well I can get a 10 gallon glass tank for $8.00 bucks - so it's like having $8.00 laying around, not going to change my lifestyle if I use it or not.
But ..... since you ask and have the tank,
Top 10 Things to do with a 10 Gallon Tank
1) refugium - you have the cheap part, you'd need a few more bucks, time and material to make one.
2) hospital tank for meds
3) QT
4) nano - again BIG bucks to make it reef, lights, rock, corals etc.
5) pod ranch
6) small FO tank or FOWLR maybe
7) snail or small invert tank
8) sump
9) macro algae grow out tank
10) terrarium or vivarium for plants or land critters