I'd play freerolls with a similar "strategy" to how you would play any other event. I don't play crazy just because I can't lose money; some people do this for the thrill, but it isn't a winning strategy long-term.
As with more "standard" games, I find one key being able to identify the other players. Who is nitty? Who is a fish? Who is shoving all-in every hand? (annoyingly common complaint many have about freerolls turned into a lottery).
Once you identify which players are doing what, then you attempt to exploit them. How you try that depends on many factors including your play-style. Be willing to adapt! Some freerolls I'm playing almost maniac aggressive and others I am playing really nitty; if your play-style is exploitative, then go for it. Just don't change just for the sake of doing something different though. If I tighten up, then it is for a reason like players shoving a lot or my raises not getting credit. If I loosen up, then it is for a reason like too many players folding to my aggression.
It isn't really a secret, but I'll play freerolls as I'd play a different stakes game; poker is poker