I agree with the previous suggestions of tank mates, but I will suggest a couple additional options. I like to refer to green sunfish as the Oscars of the native tank. A single green sunfish will develop as much personality as a single Oscar in responding to a person, moving across the tank as a person walks by, rising to greet the person, etc. If you are in the same room several hours per day, you may be all the company the fish needs. They commonly act like aquatic puppies.
Another option is what I like to call an armed tank. Three or more aggressive sunfish, the more the better, will coexist if they are about the same size and temperament. The idea is that one sunfish will not chase another because to do so opens them to attack by a third. Much of the time, they will hover and keep an eye on each other, similar to what is commonly seen in ponds. Appropriate tank mates are other green sunfish, warmouth, rock bass, bluegill, some longear, pumpkinseed, dollar, and other aggressive sunfish I'm probably not thinking of. Shier sunfish such as orange-spotted, blue-spotted, and some longear are poor selections for this type of tank. In my experience, some individual longear sunfish are aggressive and others are not. I have kept tanks such as this for several years at a time. Fish that stay too small as the others grow will hide in corners and should be removed before they are killed. Interestingly, individuals that get larger than the others don't seem to realize it and don't pick on the others.
Phil Nixon