We were in Manuel Antonio for three days two springs ago and really enjoyed it. Our hotel was the Verde Mar - not expensive, not that nice, but our room was right next to a small pool, which was great. That hotel has a direct walkway/gate to the Manuel Antonio Beach, which was a big advantage to other hotels up the hill. The location was pretty good to walk around some.
We took the National Park tour with a guide, and really enjoyed it. The guide pointed out countless animals and wildlife that we would never have seen on our own. Well worth it for a morning trip.
I agree with Judge on El Avion. We made reservations for the sunset/evening, and really enjoyed our dinner. I believe I had a cajun spiced Mahi-Mahi, which was excellent, and the atmosphere was great.
Another night we ate a more Americanized appetizer kind of meal at The Marlin, which was very busy, and good. Close to the ocean/beach on a street corner. We also had a nice lunch at another nice restaurant - name escapes me - but famous for monkey and lizard watching - FANTASTIC food there, about halfway down the main road across from the ocean, but with great views. Pretty close to El Avion.
There also was a upper shelf place way up on the hill famous for celebrities and a high society secret kind of bar, but not our thing.
We really liked that area of Costa Rica. We also went to the mountains/rain forest and the Volcano area...we were exhausted when we got back. Have a great time - you should really like it, I think.
Suggestion: Take a foam mat for the mattresses, if comfort is important to you. We wished we had.