Maybe you can make 3 types of deposit, such:
Deposit $10, get 5 free spins,
Deposit $15, get 7 free spins,
Deposit $20, get 10-12 free spins, etc.

Or you can create by yourself and your team. Or you can make Deposit $20 for 15 Free Spins on specific slots game from one provider. It is a simple way to attract people to come and play and many gambling sites use this method.

I am not good in the promotion section