Go to Monte Albán, absolutely. I'm sure you can pay someone for a tour, but there are a couple of tourism companies a few blocks south of the Zócalo that offer round-trip shuttles for a cheap fixed price (something like maybe 60-80 pesos, if I recall correctly). I would highly recommend a guided tour - if you speak Spanish, you might be able to find a guide at the site, or you can go with an organized gringo tour. In town, there are some very nice museums and churches to visit and lots of amazing food.

Mitla is another archeological site a couple of hours away by bus; it's well worth seeing. I stayed in the town of Mitla for a few nights and frankly found that kind of underwhelming - not a lot to do there. There's a massive market in the town of Tlacolula, which I highly recommend for people-watching and maybe buying souvenirs. The town of Teotitlán del Valle has lots of weaving workshops selling rugs - it's all much of a muchness, but there are a few nicer places and the town is pleasant enough for an afternoon.

All that said, I think 11 days in Oaxaca might be a lot - it's a beautiful city with lots to see, but I feel like you'll hit the main things in 4-5 days. The problem with Oaxaca's location is that it's not really conveniently located to other major tourist destinations - it's about 7 hours or so by road to the beach, and something like a 12-hour bus ride () to San Cristóbal. You could always fly down to Puerto Escondido to do a few days on the beach if you want to mix things up a bit.