Go to a hunting supply or sporting goods store and get a 30-50 foot check cord. They are very light so it almost feels like they don't have a leash on. Also:
HIGH value treats wherever her recall isn't so great. In the park, since Kai has trouble with tricks too, always keep high value treats on hand. Bits of rolled dog food, cooked meats, hot dogs, anything super stinky that she likes a lot. I've had the best luck with rolled dog food. I buy it in big rolls, cut it into tiny squares, and preserve it in the freezer. I usually keep one ziploc bag in the fridge so I don't have to thaw it but it won't ruin.
As for recall in the park:
High value treat, for one, and a long leash. Start with shorter distances. In more distracting environments, you have to pretend she's gone stupid. Lol, no offense. Make it really easy. Whatever distance she's perfect at, start there. Don't test her at a dog park close to the dogs if her recall isn't good. Go somewhere relatively low-traffic. Practice here for a while and slowly move into higher-traffic areas. Don't use the high-value treats the entire time you're working with her unless you have to. The first several trips you may have to use just the high value. Sneak in a lesser value treat once in a while so she doesn't get used to only getting the good stuff---she'll either only work for the good stuff or get bored with it.
Work your way up slowly--low traffic area first, work up to relatively long distance. THEN move to medium traffic area. You may need to go back to short distance. Most dogs do. Work up to long distance again and move to high traffic.
Keep in mind that Kai will let you know when you're moving too fast--you won't make progress. If her recall stinks when you move to the high traffic area, then you've moved too soon. Back it up a step and work a little more where she's comfortable.
Hope this helps.
If you have any more questions feel free to ask.