Grandmas Birthday Party ideas?

so im planning my grandmas 64th birthday party its going to be june 12th im hoping to have it at this beach/park im thinking around 10-15 adults not including children. i already know what cake she wants. im trying to do this with a budget but i want to make it great. is a theme party to childish? if it was like a potluck kind of party there would be a range of different foods or should i prepare everything? i dont really want the basic hamburgers hot dogs chips and soda kind of party but i dont know what would be simple and good. i was thinking of bringing the propane bbq grill. there will be a playground and the water but im sure it will be too cold to play in for the kids. what other kind of things should i bring i love my grandma tons and what to make it very special for her