Other options:
1) A game fee
2) Invitation to the pizza party that follows
3) A promise by the coach that s/he won't misbehave in games this year
4) If you're giving out team t-shirts, give one to the ref.
5) A hearty handshake and a pat on the back
Whatever you do, make sure that the team members go up and thank the ref afterwards (not as a "formal / scripted" thank you, but as a spontaneous event).
Also, consider inviting / requiring the parents to attend.