For the winter months of 2007, I putted at the upholstery buttons on the back of a lazy boy. The nice hexagonal shape was a good target. I missed high once and smashed our cat in the face, now when I pick up a disc he runs for cover.
When the weather accomodates (and in WI, it rarely does), I'll break out an old folding lawn chair and aim for the green stripe across the center. The chair itself is ugly as sin, so it's much easier to throw stuff at.
Somebody near by...Putt to a friend.
A gut shot putt usually works great!
At the NT this weekend, the practice baskets were full and there was trash talking, so one of the players broke away and got a spectator to putt at. They knew each other and he got his putting warm up done without interference.