This one is really fun and works with a girl you are just seeing (friends with benefits).
So this idea is simple and you could have your own take on it, but I think you will love it.
When you are 'seeing' a girl, a fun idea that both you and her will love. It helps if you are both already a bit kinky. So what you do is buy her a locking treasure chest and leave it at her house. Fill it with sex toys that you want to play with her with. The best part is that you keep the key. This way she will always call you when she is aroused and wants to have some fun. You can easily become her only (or at least her #1) go-to booty call. After all, there is a locked treasure chest sitting in her bedroom teasing her all the time. I've done this with a few girls and I could proudly say that I hold a few 'keys'
... And I get booty calls all the time.