They are not supposed to accept monetary tips.
Some people like to take small candies (Kisses or mini-chocolate bars) and stuff that can be shared among the Club/Lab staff. Some bring magazines that can be shared. Basically anything given is supposed to be shared among all the staff members in the Club/Lab.
From what I understand, if someone INSISTS on giving money, it has to be immediately turned in to a common pool for something for all the staff in the Club/Lab.
Honestly, the best thing you can do is mention BY NAME on the comment card and/or with a follow-up letter the staff member(s) you want to recognize. Ultimately that will mean a LOT more than shared candy or magazines or some party when there is enough money pooled together ever could.