Safe and effective: cosmetics (vehicle skins, clothing, housing decor), convenience (extra garage slots, queue priority within reason), recognition (supporter tags, credits, Discord roles), and access to cosmetic content drops.
Avoid: weapons or stats for money, exclusive moneymaking jobs, pay-to-skip progression, and anything you would not print on the join screen. Pay-to-win revenue spikes once, then bleeds your population, retention data is unambiguous about this.