Today I went thru a drive thru, put my car in park. Heard it rev up on its own a few times. When I put it into drive to leave, it did not move and the engine just revved up like it was in neutral.

Sounds like the transmission is slipping. Extremely common in the 07- 09 outlook, Acadia and enclaves. Check the transmission fluid level first. If its low fill it to proper level. If its full but it brown/black and smells burnt the transmission is junk. Call any GM dealers service department, they extended the warranty on those transmissions because of all the problems.