First of all, I wouldn't recommend walking on the street and pulling out an iPhone 5/5s/5c and start making calls. Especially at night. I'm not saying there are armed robbers on every corner and you'll get killed over an iPhone - but really you should leave the expensive smart phone in your room safe and get yourself a cheap UNLOCKED 3g / 4g world (GSM) phone for walk-around purposes. You're not going to impress anyone with it but who cares? The chicas won't care if you pull out an iPhone or an old BB Bold.
Now, your comm options to the US are varied and also depend on which carrier you use.
If you're on Tmobile and have one of the new
Simple Choice Plans, you're in luck. I use this option with my SJO phone (an old Samsung Galaxy Nexus) when I absolutely need to be in touch while out and about.
If you aren't, then your options get more complicated:
1) Either set phone to Airplane mode or at least turn your phone's "Data services while roaming" option to avoid gigantic roaming charges (as a previous poster said, $500 for a day's use is not uncommon). With the data services turned off you'll still be able to get/make calls and SMS, but at international roaming rates (another arm + leg);
2) Once phone is in airplane mode, turn Wifi (and only Wifi) back on, then use Skype or Viber to make/receive calls whenever you're on a wifi network;
3) If you don't care if your US contacts receive calls from a Costa Rican number and you have an unlocked phone, buy a Kolbi prepaid SIM card at the airport when you arrive, charge it up with 10k colones, pop it in your phone, and then tell your US contacts to call you at the CR number. You'll also be able to call the US, but international calls on Kolbi prepaid eat up your available balance quickly.
There are many other ways to skin this particular cat, but these are three you can try.
mh