http://www.united.com/page/article/0,6722,1037,00.html
If you bought anything you might get partial compensation. It's called "residual value" and even though you were forced to buy things, you now own them and get to use them until they are wornout, thus, you are also getting value out of them which is why all airlines only pay a partial amount, if any.

Get the policy IN WRITING from the agent when you file the claim on their policies when it happens. Not only with you know how much they will repay you and can budget accordingly, but you'll also know the procedure(s). It sounds like you got your bag within 72 hours, and although useless to you since you were returning that day, they met their demand. However, you should still complain, citing their "we will make every reasonable effort to return your checked baggage within 24 hours in the United States" and they don't seem to have made a reasonable effort since they knew where it was and failed to just put it on a flight for 2 days. If at the least, you might get some compensation for the items you purchased while you were without your bag. (I hope you kept the receipts!).

Here's an USAToday Article about delayed baggage on international route.

Here another USAToday Article about a lost domestic bag on US Air. Probably more similiar to your situitation than the above one.

Maybe you won't get any compensation for the interim costs? Still worth a shot.