Avellanas, Marbella are both super fun. Marbella is tough to get to w/o a boat but it's really worth it. My friend just surfed 4 Ft overhead at Ollies lasst week.
If you're staying in Tamarindo then go to Grande or Casitas. Both breaks are across the estuary. Casitas is directly across the estuary and grande is about 1.5-2 miles down the beach. Definitely take food and water if you make the walk to grande - it gets HOT.
If you do go to Witches and Ollie's, it will not hurt to bring a 1-2 mil top. Went in November 07 and the offshore 20+ mph wind and cold water upwellings made it a bit chilly.... Also, bring an additional copy of your passport for the boat trip....
Last note, when you land in CR, go pay your exit taxes (i think its $18) asap. You can pay them at any bank in CR or at the airport..... Nothing sucks worse than missing a flight because your stuck in the exit tax line on your last day!!