I think serious athletes already have strong opinions about what they want/need, and they have a solid supply of it anyway. Most gymnasts have to put up their hair, so you could consider an assortment of hair accessories - you could check at any gymnastics studio to see what they sell. It's probably overpriced but at least you could get some ideas and then look for a better deal if you think they are way too expensive.
I am really against giving 13 year olds any help in buying coffee. Aside from the fact that teens are pretty sleep-deprived anyway, caffeine is very dehydrating and just a bad idea for athletes. My son was (and still is) a serious runner, and the coaches never allowed it for minors. Even as an adult, my son rations it. Some of the teens go for those horrible energy drinks (the shots, Red Bull, etc.) and they are having heart issues, especially when mixing them with caffeine. I think kids who won't be able to afford college shouldn't be dropping $8 on a designer coffee with fancy foam either. (Same for their parents who can't make the mortgage.) I think it's encouraging a high level of consumerism, and I'm opposed to its message. I love the idea below of putting a gift card in a coffee cup, but I wouldn't do it for a young teen, just an adult or at least a college kid.
I know what you mean about trying to keep the dollar amount under wraps. If you do a gift card, I'd do $20 or $25, never $50 for a casual friend you don't even know. That's ridiculously unsustainable if kids have more than 2 friends. For graduations, I tend to give $20.19 (last year, everyone got $20.18!) because it's memorable.
Something like earrings is pretty standard, and you should be able to find something she likes that won't break the bank. But if your kid is opposed to it, and these friends just want to exchange cash back and forth, I guess you can go that route, either with a gift card to a store or a Visa card.