R.W.
Have the girls put their swimsuits under their dresses for the limo ride to the beach. Once at the beach go to the cottage first and have them leave the dresses there. Go have fun on the beach. Then go back to the cottage and let them get dressed back up for dinner at the cottage. If the limo is to much then maybe you could rent something for the day. If it's on the weekends the rental companies have great weekend rates. You don't say how many girls she wants to take, or metion her age. A Hawaii party would be fun, they could wear grass skirts over their swimsuits. Instead of vollyball, etc. you could do hula dancing. An instructor for an hour doesn't cost that much. The limbo would be fun. The shell game. A scavenger hunt. Look online for ideas. You will be surprised at what you find. If she's old enough give her a limit on what she has to spend and let her plan the party that way your off the hook if it's not exactly what she wants.