I am the "using up leftover lettuce" queen because my CSA (community supported agriculture) group always gets a LOT of lettuce.
Lettuce soup is good, believe it or not! Chop up the lettuce and cook it until it's very soft in some chicken stock (or vegetable stock) with some garlic. Blend it in small batches. The only lettuce that doesn't work well with this is butter lettuce (too slimy when cooked). Be sure to season it heavily (salt, pepper, cayenne).
Braised lettuce is yummy. Put a little olive oil or butter in a big skillet, throw in roughly chopped lettuce, put a lid (or foil) on for a few minutes until wilted, add plenty of salt & pepper, and a little thyme if you have it. If you have some frozen peas, you can throw those in with the lettuce. They don't take long to cook and give this a nice mixture of textures.
If you don't want to cook it, make lettuce wraps. You can wrap almost any sandwich filling (hot or cold) in lettuce leaves.