Most non-impact toys are safe as long as you pay attention to your partner. For example, if you tie someone, are their fingers getting blood or is the gag causing them to dry heave. For impact toys, start with something reviewed as mild and make sure to practice on yourself first.

Edit: And of course the obvious, make sure you communicate what you want to do with your partner. Having them raise objections ahead of time, and possibly demonstrating, solves a lot of issues before they come up