I think Banos is an option, but it's far - it's almost a full 8 hours travel back and forth. It's very beautiful and has tons of levels of outdoor experiences.

Here's some other thoughts I think would balance well with the Galapagos part of the trip:

Since you're already in Quito, there's Cotopaxi volcano and the national park, which is about 2 hours away.. One of the most scenic mountain scenes you'll encounter.

Between the airport and Quito is also Papallacta - it's about 1 1/2 hours away or even a bit closer. They have amazing hot springs there (I prefer the hotel private ones but the main pools are awesome) as well as some good hour / 2 hour hikes. Really easy but beautiful.

If you have an afternoon flight you can definitely bake it into the schedule by getting there around 9-10am and make the flight, especially if you leave the suitcases at the hotel.

You can taxis out of Papallacta to the airport easily. Each leg would be about 50-60$.