What about Monteverde, if you have not been there yet? To cool down a bit, and see another type of nature.
Another choice would be the Alajuela area. This is where the airport is located so convenient to that.
From Alajuela, you can visit Braulio Carrillo National Park, go to the Orchid Garden http://www.orchidgardencr.com/ , head to Poas Volcano or Irazu Volcano, visit Zoo Ave, do a coffee tour, arrange rafting (if you haven't done that at Arenal), a day trip to Sarapiqui for the La Selva Biological Station - lots to do there. And lodging of all sorts and all prices from hostels to the beautiful Xandari -- and everything in between.
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