We stayed at the Fiesta Premier Resort for two weeks in August / July. Though you wouldn’t be able to tell from the photos (see link below) we got bored after so much time there, despite taking four days to do tours off-site (5 days if you include the 4 hour ATV tour).
With that said, we would highly recommend breaking the trip up. Perhaps one week in the Arenal region, and the second (more relaxing) week at the all-inclusive Fiesta. What this will do for you is make some of your day excursions MUCH shorter. I can’t remember the length of the trip to see the Arenal Volcano (3 hours? 4?) but it was a long one. There’s lots to see in that area that we simply couldn’t do because of the distance. So one week there, one week at Fiesta seems pretty ideal. We came across a few couples who had done exactly that and they seemed quite pleased with their decision.
Hope that helps.