Dear {Recipient},
I probably don't tell you often enough, but I really appreciate you being in my life. Thank you for always being there for me, especially when {significant moment}. I'm not sure if you're aware, but you've really helped me through a lot.
I'd like to do something for you for a change. Is there anything you need help with right now? Or how about if we {grab lunch/catch a movie/etc.} this weekend? It'll be my treat.
Love,
{Your Name}