That's a good question, sometimes it is very hard to think of ways we can show our Mommies and Daddies that we love them and all that they do for us. Admittedly, even at the best of times, my regression and baby time with Mommy can sometimes feel one-sided, I subject her to my diapers and other baby needs and she goes along with it without batting an eye, I'm lucky to have her, but as much as I enjoy our time together I often feel bad because the nature of our play is me sitting back and being cared for while she does all the work.
I've talked with my Mommy about how I can show her my appreciation and she told me that she is happy as long as I'm happy and that if I really need to show my gratitude small gestures are the best way to go. So every time we finish having Baby and Mommy time, I'm always sure to thank her with a big hug and I make sure to tell her just how much she and what she does for me means to me. I've also showed her my gratitude through small gestures like she suggests, I bought her a card for Valentines day, I've spotted her gas money to cover her trips out to me, I've drawn her pictures and sent her small friendly texts and I've paid for and taken her to dinners out on a few occasions to show my appreciation. So, I guess what I am trying to say is, let your words and actions show how much you appreciate her and if that doesn't suffice, then do small things like dinners out and tiny gifts.
I think many of us AB's tend to overcompensate when we find someone who accepts us and is willing to indulge our baby side, like we feel we need to give them the world in order to make them stay, I've thought that way with my previous girlfriends in the past, but with my current Mommy things are different. I asked Mommy what she would think if I bought her jewelry and expensive things, she told me flat out not to, that she doesn't want to feel as though she is being paid for a service, " I'm your Mommy not a prostitute", is what she told me. She says that she genuinely likes taking care of me and that a gentle smile when I'm in baby-mode and a nice big hug and a thank you when I'm put back in my big boy clothes is enough to let her know that I sincerely care about all she does for me. Now, every Mommy is different mind you, if you don't think words and small gestures are enough maybe you could ask your caregiver point blank what it is you can do for her to show your appreciation. Best of luck to you