I'm pretty cheap, but that doesn't mean I don't know how to have fun. Cheap fun and romantic things that I like doing are:
Learn improv together: there are some places that offer free classes its extremely fun and will help with your quickwittedness tremendously
Outdoorsy stuff: whatever you are into, ultimate frisbee, walk in the park, feed some ducks(throw the bread in between her legs its hilarious

) google waterfalls near you and you can find some waterfalls to take her to, discuss philosophy, ice cream, walk around downtown
wow I could name off a shitload of these the options are limitless. Find something thats easy for you to setup and sounds fun. Heck I created a slip and slide in walmart with detergent once just for the hell of it. Do all that crap you wanted to do as a little kid when you turned an adult, but still haven't done. Just enjoy yourself man and share that joy with her.
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