I dont have staff so i cant suggest anything for them but, ive recently started a client loyalty scheme whereby every time they come and spend over £25 they get a smiley star sticker on the back of a business card, when they collect 10 stickers they get a free infill. It also ensures they have your number in their purse to give to people when they're asked 'who does your nails' ! I also tell my clients they will get a free infill when they introduce a friend to me, once that friend has been for 3 treatments.
Sometimes I do salon bingo! I drawn out a card of squares, say 3 by 3 and I put a task, treatment or retail product in each square. If the therapist gets a line, she gets a small reward and if she gets a full house then she gets the star prize!
It's up to you what the prizes are and if you run the bingo over a week, weekend or just a day. We have great fun with it and the first week I brought it in, the salon profit nearly doubled!