Most Helpful Girl
It really depends on what's available in your area!
If you're in the city, you could take a stroll through the nice part of town holding hands and enjoying the lights. Lit up bridges are one of my personal favorite spots.
If you're by a beach, pack some beachwear and go... do what y'all do whether that be stargazing, swimming or getting it on.
If you're in a more wooded, quiet type place then I recommend a bonfire and stargazing.
Those are just some things I personally enjoy, don't cost a cent, and is private, special and romantic.