A couple of months ago, I started participating in a government nutrition study that is meant to correlate the relationship between eating habits and overall health. The first part is a bit time-consuming because there are three days where you will need to fill out a food diary, but after that, they only send one short survey per month.
There's also an opportunity to do a free health check-up at a local clinic. I did mine last week, and they drew four vials of blood, took my vitals, did a short strength test and then measured my body fat and BMI. If you've ever wanted to know exactly how much each leg/arm weighs, than this is for you. :) I'll also get my cholesterol results by mail in a few weeks.
Besides that, there isn't really any "reward" for participating, but I like the idea that I am contributing towards health research. They are still looking for other participants, so if anyone is interested in signing up, you can do so at: https://www.etude-nutrinet-sante.fr.