Print, Laminate, and hang up on your board for early finishers! I use magnetic letters for mine, but you could easily write your letters in with dry erase markers!
I leave my letters up for one week. During that week, as students finish early, they can write the words they find with the letters. The letters have to be touching each other for it to count. Later in the week, you may allow them to use ANY of the letters in any order to create words.
--I do not take credit for the idea of Boggle, but have simply adapted Hasbro's copyrighted game for the purposes of my classroom--