I can't help with the Arenal area, but have spent time each of the last 6 or 7 years at Los Suenos in Herradura.
You should definitely do a zip line/canopy tour. Some of the ATV tours are also fun if you want to head off the beaten path up into the mountains and don't mind dustry roads. There are also a few companies that do excursions to an island up the gulf from Los Suenos - I think it's called Isla Tortuga. I've always been preoccupied with surfing and fishing when I'm down there so I haven't had a chance to check that out personally, but I hear it's worth the trip if you want to relax and do a little snorkeling.
If you're interesting in fishing, I've been a client of Dave Mothershead on the Miss Behavin' out of Los Suenos for years. You can check him out online @
www.costasportfishing.com. He's one of the best charter captains I've ever fished with. When the bite is one, Los Suenos is a world class destination for big Pacific sailfish with blue and black marlin mixed in as well. Local restaurants will also gladly prepare any fish you catch if you decide to go after meatfish like Mahi Mahi or Snapper.
As far as dining in that area goes --
El Galeon - fine dining on the marina at Los Suenos. It's probaby the priciest place in the area, but it's worth it. I usually make it a point to eat there twice a trip.
Juanita's and El Rey Marine - Both are located right on the beach in Herradura, outside Los Suenos and serve local style seafood. Great place to order a whole fried snapper or grilled lobster.
Aberdeen Angus - Great little Argentinian steakhouse in Jaco. Best bet if you're craving beef.
Pacific Bistro - Great seafood restaurant in Jaco. It's towards the North end of town across from one of the newer shopping centers.
Pizzeria Bocha - Pizzeria run by some Argentinian's on the main road in Hermosa. Great pizza.