I thought Quito was very cool, but yeah 5 days is a lot. In addition to what Gecko says, you can go to the equator which is cool. Wife and I did all of that in one day. We hired a car out of hour hotel who took us to all those places for $120. Probably could have bargained down to $100 or maybe less, I think that's what you're supposed to do, but whateves. The dude was totally cool. Ecuador uses dollars so no need to change money.
Actually, we only went to Quito as a gateway to Galapagos. Galapagos is pretty controlled, you can only visit most of it if you're on a tour. Sounds like all your tours are booked which is good, especially if you're only traveling as a couple. Last minute stuff can be tricky, we lucked onto one tour because the capacity was 12 and there was a group of 10 older Germans. (The krauts were by and large very cool, one bitchy lady trying to be in charge but even the krauts ignored her. A couple of them were familiar with UT and our football team, they started the convo due to my longhorn cap. This was August of 2017 so I didn't say much about football.) In my experience, the tour guides were all very knowledgeable and educated. One of our guides had a Masters from Cornell. I think all the guides are gonna speak English and most hotels and even restaurants will as well. Spanish comes in very handy of course
If you're staying on Santa Cruz for any period of time, there is a street market where you can grab dinner. Los Kioskos - basically a bunch of different outdoor restaurants. Seafood is the specialty obviously. Kinda fun to walk around downtown Santa Cruz, seems pretty safe and again, they control who can go in and out.
If Quito gets boring, Guayaquil seemed cool, it's actually the largest city in Ecuador. But I never actually visited, just stopped at the airport.