A few suggestions for lodging - Pachamama in Punta Uva and Banana Azul in Puerto Viejo. Both in similar, moderate, price range.
You would probably want to plan more than a night or two in Arenal. It's quite a drive...I'm not sure how long the trip would be, but I think it would take over 5 hours to get there from the Puerto Viejo area.
Maybe consider spending 5 nights on the Caribbean side, then 3 in Arenal. There is a rafting trip offered through Exploradores Outdoors that will provide you transportation to/from Puerto Viejo and Arenal before and after your rafting trip. So, you could start the trip on the coast, then they would pick you up at your hotel, go rafting, then drop you off in Arenal. It's $99 per person, including the transportation. Just a thought.
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NEWS AND BLOGS ABOUT CARIBE SUR - THE OTHER COSTA RICA - Puerto Viejo, Manzanillo, Cocles, Chiquita Village, Limón, Costa Rica