>>anon22+(OP)
Just say "Eid Mubarak" with a warm smile, should be good. Could be followed by a friendly hug and a gift of sweetmeats but that's mostly for family & friends.
And for the curious: the proper response to an "Eid Mubarak!" is "Khair Mubarak!"
>>Toobam+c2
"Eid" is the day, and "Mubarak" means merry/blessed. So "Happy Eid" could be the sentiment if you're uncomfortable with pronouncing "Mubarak" (which is completely phonetic, by the way) or just "Eid Mubarak."