To elaborate on why Golden State would do this: I believe they will be looking to unload Lee's salary so they are able to re-sign Draymond Green. Also Klay Thompson has a contract extension that will kick in soon.
I think it would be best if a Ainge finds a contract that fits within the 12.9 mil range we have for the traded player exception (TPE). McGee works, but I have some doubt that Denver would be willing to offer a first rd pick to get rid of him. They are not contending and their 1st rd picks would be too valuable to trade.
With that said, maybe Ainge can squeeze out another "conditional 1st rd pick" that turns into 2 seconds. In return the Celtics take on the immediate cap space and send two serviceable role players on expiring contracts to Denver in the process.
Bottom line: any team which possesses a player who has 2 years left on his contract (or 3 yrs w/ the final year being a team option) could offer Boston a conditional 1st rd pick for expiring contracts. The player can't be too talented (i.e Nicholas Batum) unless Ainge is willing to give up assets in the early stages of this rebuild.
So essentially the team trading with the Celtics would be getting expiring contracts & a trade exception up to 12.9 mil. That trade exception could be valuable to a team like GS who might be looking to add another player in the offseason. With the TPE they could add another player w/ a large salary or two players w/ smaller salaries.