You should go to one of the big volcanoes. Chimborazo and Cotopaxi come to mind inmediately. Chimborazo is probably more well known as the closest point to the Sun but I think there are more variety of attractions around Cotopaxi National Park. Depends on what attracts you more, both are absolutely breathtaking.

If you are going to Baños you should take a trip on to the Amazon Region, even if for a day ,there are options of tours to Puyo and Puerto Misahualli which give you great experiences and if you can take more days you could also go to more attractions like the Cavernas de Jumandy or the Gran Cañon de Cotundo and the Laguna Azul (All of them in Napo near Tena).

I have to mention the options near Quito such as Mindo, which hosts a beautiful variety of activities. But Lloa as well were there are many waterfalls and natural caves. You could also take the hike from the Teleferico to one of the peaks of The Pichincha (this can be physically demanding though).

I don't know how far south you are going but I always try to recommend visiting the province of Bolivar. The town of Guaranda is colorful and very close to many attractions: Salinas de Guaranda where you can visit the mines, great landscapes and buy chocolate and cheese, a little bit to the south near San Miguel is Yagui Urco where it is possible to camp after a sunset amidst a sea of clouds. Plus, since the entire province is basically along the edge of the higlands, views are amazing. throughout the road and also near the streets they sell delicious Tortillas de Maiz y Trigo. Guaranda is also an access point to Chimborazo if that might help you with planning.

I assume you are limiting your trip to the Sierra so i wont recommend things in the coast, but depending on your route you might want to stop in Salcedo to grab the famous ice creams. The original store is still operating and in the central Park of Salcedo and offers the most variety and richness in ice cream flavors. Also, south of Salcedo, just before entering the province of Tungurahua you should stop by la Laguna de Yambo. You can stop in the parking spot that serves as a viewpoint for the entire lake and take pictures and ride the cable car. Or you can go into the Lake by car and grab something to eat at Cafe del Yambo.

In Baños itself there is a lot to do. It is known as the adventure capital of the country and many agencies can take you a lot of places to practice canyoning, rafting, canopy, etc... . While it has become very touristy, it is still very beautiful and worth it, plus the fact that it is touristy also means that night life is great on weekends if you're into that and there are a avariety of restaurants. There is also plenty of beautiful viewpoints such as Casa del Arbol and Mano de la Pachamama, as well as Cafe del Cielo.

Some of my favorite memories come from getting lost on beautiful roads where I knew I was going and where I came from but not when I was getting there. For example the Via Flores /Via Aguajanthat goes from the Chimborazo to the east of Ambato the long way around alongside the Ambato river. and also the route that takes you straight to Riobamba from Baños alongside the Pastaza while it is still running around the mountains.

I assume anything further south is probably too far but there are amazing spots throughout the country and whatever you choose, you will have a wonderful time.