The problem with offering tangible rewards to increase participation is it usually has a negative long-term impact.
Members that were previously making contributions for internal rewards (appreciation, self-identity etc…) suddenly relate their contributions as part of a transaction. The best transactions mean getting the most and giving the least.
In addition, the impact of rewards is diluted over time. So you need to continually up the reward to maintain the same level of participation.
Don't use rewards, it's not sustainable. Certainly don't use defined rewards for a defined amount of activity.
Instead, if you feel a key member's commitment to the community is sagging, send a one-off thank you gift. Be clear that it is a thank you, make it personal and give no indication that it will ever happen again.