For my developmental girls I use sticker charts. One for new skills, so they get to put the sticker on the chart, high fives, and recognition.
I also have a conditioning chart which has 12 designated strength/flexibility milestones. They get a sticker for each one they reach and a prize for every 4 stickers. I have a prize bag with gymnastics stickers, headbands, socks, water bottles, cheap gymnast necklaces, 5 min. trampoline time pass, you pick conditioning pass, anything gym related I find inexpensively. They get a "big" prize for checking off the whole chart- a leo or gym teddy. I have collected a few $10 leos and made a few others myself and I make the gym teddy bears, so it's fun for me to make/collect the prizes and the girls enjoy it.
I might make up some excuses for popsicles this summer because our gym is unbelievably hot and the girls work so hard. We have one girl struggling on her pullover (the only one), so I might bring popsicles when she gets it. Bhs are a big goal for the group too, so hopefully sometime during the summer they will all have that to celebrate!