You can, but I advise against it. Even if you pay in US dollars you will get your change in colones. You will get the best exchange rates at the bank, all you need is your passport for ID. Exchanging at the bank ensures that you don't get ripped off by someone. Not a lot of people realize that the exchange rate has changed. 35 colones is not much, but it does add up quickly. Check with your bank before using your debit/credit card. I have a bank in the US and every time I use the ATM here it is a $5 fee, plus the cost of the currency conversion which is a percentage. Some banks here charge you for using their ATM and some don't. One bank I use will charge me at first and then reverse the charges later. Thank is Banco National, Banco de Costa Rica has never charged me, just a small conversion fee from both banks, and I do mean small, like $1.

Yes. US dollars are accepted everywhere, and you usually get a slightly better exchange rate just paying with dollars versus exchanging at the airport.

The short answer is yes, dollars are accepted everywhere. Sometimes things are even priced in US dollars.

Some places will not take bills higher than a $20 because of problems with counterfeit money.

I usually get like 40000 colones out of the ATM at the airport.. just so I have some with me. If you pay with US dollars, you will get change in colones.

Just remember, ROUGHLY, a 20 dollar bill is 10,000 colones. (technically its more like 10500, but it's easier math to ensure you don't get ripped off to just remember 10000=$20)

Also, bring a credit card without a foreign exchange fee, as that will give you the best possible exchange rate. Most of the country will take credit cards, but there may be some remote areas that won't take them, so just plan ahead.