Don't rely on the ibuprofen alibi. The length of time or a THC positive varies. The average time for the regular user is less than two weeks. Water is a good method for these people, as insurance. An infrequent user should not have to worry after a couple of days.
Substitution is the most interesting and challenging approach.
IF you really hate the Bladder Cops, this can be quite gratifying. If you are using a friend's urine, be sure you know what drugs they have taken over the last month. There is no definitive test for amphetamines, OTC remidies such as Contac and Alka-Seltzer are notorious. If you don't list it, most any drug can get you into just as much trouble. For any substitution be prepared to give 60cc.