How do I show this girl I love her?

Me and my girlfriend have been dating for about 8 months now and last week she left to Punta Cana for summer vacation and she comes back next Tuesday and I want to surprise her the day after she gets back with a cute/romantic date. I don't want to go to a restaurant so maybe more of the likes of a picnic at night and watching the stars or something. I would also like to be somewhere where we can be alone and secluded and I live in the chicago land suburbs. what would make it better?