I use this for my students' science vocabulary words, but it can be used for vocabulary words for any subject. With this contract, students have the ability to choose what interests them from a range of activities. Each activity is assigned a point amount depending on its difficulty and the amount of effort it requires. You, as the teacher, choose how many points students must earn to hold up their end of the contract. (The required amount I use is 75 points.)
I decided to keep this a word document so that you may change it as needed.