travelers checks

Yes, you can cash them. Some places may want to give you a slightly lower rate. You don't have to accept that; you can take your check to others and see if you can do better. There are lots of easily found places to change money, especially in Sosua.

Ken is right on the money, no pun intended, most Cambios will meet the tasa(rate) for cash,with AMEX Cheques, some are a few pennies lower.

I have my 2 or 3 regular places that do not even ask for ID any longer.
You will need an original passport at an actual bank, or make a copy of your passport and leave the original in the safe to use at a cambio.