How about carrots?
They add a bit of sweetness like red peppers and of course color. Try slicing them into matchstick (or a little bigger) sized strips. Brocolli stalks (save the florettes for something else) cut the same way would add green - just don't over cook them. And tomatoes that have been well deseeded so you have only the flesh. Roma tomatoes work well. Cut them in half, squeeze each half to eliminate the seeds and plup.