As popularised by Morning Brew, stickers and badges are low-cost, popular choices for your first physical milestone reward. Normally between 3-5 referrals.
They work especially well if you have a technical audience and a strong brand.
But don't make the mistake of just slapping your logo on a sticker...
For stickers and badges to be effective rewards that motivate your audience to share, you need to put in a bit of effort and creativity. And come up with something pretty cool.
Try to come up with some fun, unique slogans. Then use a service like Fiverr to hire an affordable designer to add some illustrations and make it pop.