Branded stickers tend to underperform expectations as a referral reward. They're also a pain (and not free) to distribute. That said, people do love receiving physical rewards. Consider using stickers as a reward for 2-5 referrals if you have a strongly branded newsletter. Especially if it's meant more as a gesture of thanks (rather than an incentive).
⏱ We're still collecting examples of this reward type. Check back soon!