I'm a bit dubious about the toilets, it's a bit difficult to do have sex in a standing position (in my experience) and it might be a bit icky otherwise, how about a picnic rug or sleeping bag on the beach or in the dunes behind the beach? The closer to the party it is, the easier it is to lead her there.
Well, assuming you can get a makeout, do everything the same and let her think you're going to make out with her (strong eye contact, lean in a bit, touch her body, face, hair... breathe some hot breath into her ear)... when she's excited then just say "I have an idea" and lead her dominantly to the seduction location. What do you think? I am just making this up as I go along, there are many ways to isolate / escalate.
How about posting an FR in which you tried to isolate / escalate and I will try to think how you could have planned it better. But honestly, you sound like you're doing fine, just keep in mind that public makeouts don't lead to sex unless you pull away real quick and leave her wanting more.
Ray