Usually for long term tracked payments (if they can be revoked - eg: user cancel suscription before 1 month) you can either delay rewards or if reward is immediate and final (but your earnings are paid on the first of each month, for example) you could get it out of your own pocket/have extra funds available.

A membership portal with various merchants available with variable cashback per $ spent or per completed offer would be great.

There are also the "earn free reward" networks with extensive API which offer various offers like surveys or subscriptions (the kind power free facebook credits, free Kreds on Kongregate's gaming portal, free gold, coins or any website's currency. Those could also be applied to reward members but with bitcoin rewards that can be cashed out.

Usually the more merchants/free rewards/ads provider can be included/offered, the better (may require implementing many external API as well as a in-house one for direct advertising merchants). It would require a members portal in any case however.