There seem to be 2 types of incentives in contracts: "Likely to be earned" (LTBE), and "Not Likely to be earned" (NLTBE). Pages 109-124 of the NFL Collective Bargaining Agreement lists all of these incentives (as well as rookie incentives broken down by their draft round), and it looks like they are the only things that can be offered. I also found something here (although a little less official) that says essentially "anything as long as it's legal". Although this citation admittedly leaves more to be desired as far as an explanation and it's more specific to endorsement contracts.