I build her a lot like Irelia, with Trinity Force and tank items. I opt for a Warmogs rush for your first item, you'll farm is up fast. Then work on your triforce, starting with Sheen and ending with the Zeal. That is pretty much your core (+ boots of course, I prefer CDR or Merc Treads, depending on the enemy team).
For summoner spells, you'll want Cleanse/Flash so you will be uncatchable. If your team is lacking Exhausts or Ignites, you can replace Cleanse. I run 9/0/21 masteries for even more movespeed and CDR.
This is how this build works:
You do pretty much all your damage in cougar form. This requires you to be all up in your enemy's face, and if you're CCed (stunned, silence, etc), well you're going to die. You can play her AP, where you poke with your spears and heal for a lot. But that's not manly enough for me. I can go in their and spam all my skills in cougar form and get out to heal and hop right back in. You will be able to beat AP Nidalees 1v1, and possibly AD Nids. Give it a go some time if you have her :P
|Here's some fanart by Ganassa,|
he has other swimsuit pictures of other
champs too so check him out.