Triathlon
Watersports are always good, however expensive
Climbing is another good one.
For the cheaper options, this might depends on any equipment you have. Outdoors is good so cycling or jogging could work, so could walking - all good for just couples, perhaps also tennis.
Then there is the option of joining a team sport but I'l guess just sports for the 2 of you. Soemthing like cycling, jogging or tennis
softball
walking
running
Football.
Running is always great for couples thats what i always use to do when i had a girlfriend, go to trifind.com to find runs for both of you to do. (Look for custom search on the left) and there's a ton of options to pick from
Me and my gf are active people so any ideas? Also we are 14 and 15 so nothing inappropriate please
Also we live in a town with one really big park/reserve so thats the only place we can really go
Thanks