There are all kinds of things you could do:
* each referral earns a straight dollar value. Clients can request payouts when their "bank" reaches a certain dollar amount.
* each referral earns a "point value", and those points can be applied towards items in a "reward bank" you set up. (Keep the bank relatively small, though - and if I were you, I would not include branded items in that reward store. Branded gifts are great to send to clients for being with you for a long time, for referring (say) 20 clients, or something else. They shouldn't have to spend their reward money on something that's got a company logo on it.
* clients can apply their referral bonus toward hosting services, withdraw the money, or have it donated to a charity (or a community group) in the clients' name.
Don't make it too complicated - you'll only give yourself and your clients headaches. This is supposed to be a reward program, not another chore (both for you to maintain, and for them to choose their reward).
As to specific rewards/point structure...I'll leave that up to you to figure out. You can tailor it to best match the ability of your business to provide good rewards and not financially hurt itself, and also provide something your clients will really appreciate (or even have fun with). I can't judge those factors for someone else's business, though.