If Guardians is off your list, I think Iron Man should be as well. It's very intense at the begining. Then Tony Stark is...not exactly a good role model to begin with. He's a genius playboy billionaire, by his own admission. Mphasis on the playboy.
If you want to pick a Marvel movie but are worried about content, start with Spider-Man: Homecoming. Great movie, you don't need a lot of background to get the premise, and there's not really a lot of objectionable content (some language, but it's infrequent and comes and goes). Not super intense violence wise either. The fact that Peter's still in high school probably helps.