Frontiers of Flight museum at Love Field. C.R. Smith museum near DFW airport (although I've never been there). Also, DFW airport has a great flight watching location on the north end of the field, google the Founders Plaza.
I know you asked specifically about aviation, but I would mention the 6th floor museum at Dealey Plazza. It chronicles the Kennedy assassination. If you are a history buff it is interesting. I try to spend an hour or two there when traveling through Dallas.