I'll chime in and agree with the others – approaching people in the park isn't a great idea for many reasons. It's kind of creepy, you're not likely to find customers who later decide to hire "that guy in the park", and it's just not very professional. If someone calls to complain (which someone eventually will), the park will probably kick you out and possibly issue you a citation for not having a permit. Not a good thing for your portfolio or reputation!
For your portfolio, you obviously want the most beautiful images possible. You've already found a nice location which is a key element. With a strong composition and proper lighting, you're another step closer. Now you need to find just the right person or people for your shot. It's highly unlikely that they'll be randomly walking by the moment you're ready to take the shot.
Look to a friend who has the right look or possibly consider hiring a model. You can find candidates on Model Mayhem or other sites who will work in trade or inexpensively. You'll also have more control over their clothing, hair, and poses. A model building his/her portfolio also has a vested interest in getting the shot to look as great as possible.
Now that you have your location, lighting, composition, and model completely dialed in, you'll be creating shots that you'll be proud to have in your portfolio and will get you more paid work. (This may be contrary to what I said previously, but if you're shooting in the park with a model on a single occasion, you can probably get away without getting a permit. Just keep a low profile!).