If a CM is not one of the 4 tipped positions they are not allowed to accept cash or cash equivalent gifts (phone cards or gifts cards).
Individual snacks are always well received - candy, chips, pretzels, peanuts, raisins, fruit snacks, juice boxes/pouches, gum.
We left small boxes of raisins and a pack of juice boxes, along with a few other goodies and we got a very good reaction from that.
If you know the CM's names that you have had a good interaction with make sure you list their names on the comment card. You can also leave a note with guest services or in the comment box if you have too many names to list on the comment card. Those positive comments go a long way for CM's and they can get better schedules, extra time off, special meals and other incentives.