I've added a pureed spinach layer to lasagna. If you don't add salt and garlic, it will be very bland.
We've recently discovered that sauteed kale (flat Tuscan-style, not curly) is an even better layer in lasagna. Stem the kale and slice thinly. Saute in olive oil on medium heat with lots of garlic until limp (~5 minutes), You want to cook it long enough to soften the kale but not so long the garlic begins to burn.
Also, shredded carrot or shredded zucchini with raisins can make tasty muffins.