This depends entirely on the mid. My son isn't a spender, so we have only paid for air tickets home and an occasional care package. If mids/plebes go into town on Saturday liberty, they have the opportunity to eat out, go to the movies, etc. On the yard they can purchase items from the mid store, but they were given everything they could possibly need during plebe summer and that has lasted through the AC year to date for my mid. He gets his haircut, laundry, etc all done on the yard and those things are automatically deducted from his pay--whether he uses the services or not.
According to FB posts from other parents, some mids find lots of avenues for spending, so those parents are sending money or putting money on the yard card on a monthly basis. I'm sure someone else can elaborate on what and where they are spending the money.