On a day-to-day basis, carry anywhere between $50 and $200 U.S.D. in small bills. Most places won’t take $100 U.S.D. bills, so if possible, change your money to colones and carry around C. 1000 and C. 2000 bills (more or less $2 and $4 respectively). Carrying small bills will make tipping and purchasing small things all the easier.
Remember to play it safe and not run around with loads of cash in your pockets. Costa Rica is a very friendly country, but you don’t want to run into any unpleasant surprises. Keep any cash and valuables you won’t be using in the safe in your hotel, and avoid counting money in public.