Los Suenos Marriott near Jaco beach.
Jaco Beach is the big surfer hangout, but the quality of the sand isn't the best.
The best place is near Guanacoste on the Pacific side just north of Jaco Beach. It's ranked as one of the top spots in the world, with perfect sand, etc. There's a Four Season's Hotel there, but I hear it's like $1000 USD per night. You could stay across the street in a condo like place, which has use of the Four Season hotel pool/beach/amenities for much, much less. One of the gals that works for me did that.
At the Los Suenos Marriott, it was only like $135 USD per night. It's a 4-5 star hotel with a huge pool that's like a maze. You can feed giant iguanas and lizards french fries, etc like we did. From the hotel, you can take about any excursion for a reasonable price. Heather and I stayed there and went to Arenal.
At Arenal, you can go to the Tabico Hot Springs and sip some drinks while watching fire/lava/rocks come down the erupting volcano while sitting in the volcano heated water/springs.
Costa Rica is well known for the zip lines and mostly for what some say is the best white water rafting in the world. If you stay in San Jose (capital city in the mountains) you need to have dinner at the Monastery. It's an old Spanish monastery that overlooks San Jose up in the mountains, amazing view and sometimes live music/bar scene. The drive to the beach near Jaco or Guanacoste is only like 1.5-2 hours from San Jose. English literacy is like 94%. They're forced to learn it all through school. Almost every restaurant has menus in both English and Spanish.