If you're taking the Mt. Rainier Gondola, I recommend eating at the Summit House Restaurant while you're up top. It's a charming spot just steps from the Gondola, offering a warm and inviting atmosphere, American cuisine and a selection of fine wines. It's not the cheapest, but it's reasonable considering the location!
Dalt's Pro tip: Make reservations close to sunset and request a seat terrace side. If you're lucky, you'll be soaking in views of Mt. Rainier and savoring a delicious meal at the same time! Just be mindful of the last gondola down times during your planning!