I guess it depends on both how much you are willing to spend and how many clients you have.
I personally try to keep it health/fitness related. In the past (for my long term clients), I’ve done the following ideas: a free session (they really appreciate this), small gift card for a place they like, a set of gliding discs, water bottles, and even a foam roller. I also like the idea of contributing to a charity. Hope this helps.