For my 2 year old just getting a sticker (thomas the tank engine) was enough. Although to be fair after one week he didn't want the sticker but still did what we wanted him to do (sleep in his bed not ours!)
Can he chose dinner? Or lunch? Or where you go out?
I did that recently with my DS1. I didn't know what to do that day so told him he'd been a good boy (true) and he could choose where we went. He chose to go back to a really good park we'd found earlier that week. Nice and easy for us and he was so chuffed to have chosen our day out!
Does he like the baby? When DS2 was born, a real treat for DS1 was to be allowed to cuddle him (DS1 lay on his back and I put DS2 on his tummy). So I said if he stopped trying to clmib on us whilst DS2 fed, he could have a cuddle afterwards. And of course got loads of praise about what a nice brother he was.
In fact, thinking about it, DS1 is quite easy to "treat". Eg. he loves having his baby brother in his bath so I sometimes "allow" it
or he loves having him in his bed so I let DS2 lie next to him whilst I'm reading the bedtime story.
Actually, back to the stickers, I found that after a week of DS1 earning his stickers for sleeping in his bed, we pulled a big surprise present out (leftover from Christmas!) and that made more of an impression on his behaviour than the stickers did. So although it was only a one-off reward, he kept up with the good behaviour IYSWIM