My oldest daughter had a Webkinz Party for her 6th birthday. Here's what we did (please keep in mind that I'm a cheapskate):
We copied an image from the login page of Webkinz.com and used it to make our own invitations and thank-you cards.
We rented a pavilion at our local park. To my husband, it was worth the $$ to not have to clean our house both BEFORE and AFTER the party. (We have 4 kids...we can't seem to ever keep our house tidy.) We made the covered pavilion into a pseudo-Webkinz World. There were several picnic tables there, so we bought a multicolored "Happy Birthday" tablecloth and several solid-colored plastic tablecloths and had each table be a different "area" in Webkinz World. I made posters (again, using different solid-colored posterboards) to advertise the areas. We had an Arcade, a Tournament Arena, a Clubhouse, and a Curio Shop.
1) Clubhouse. This is where we set the presents and cake. This party was before the figurines came out, and my daughter LOVES chocolate, so we just made a huge brownie and decorated it with rainbow sprinkles. It was a lot cheaper than a cake and all the guests loved it. When each guest arrived, we'd have my daughter open their gift and take a photo of the two of them. (These photos were included in their thank-you card.) Then we'd give the guest a paper bag with a W outlined on it. Each child got to sit down at the table and decorate their bag with markers. These would be to hold the prizes they earned. When they were done decorating, they could move on to another area and choose an activity. Everyone rotated around.
My daughter and one of her friends posing with his gift:
2) Arcade. In Webkinz World, this is where the games are. In particular, the Wheel of Wow and the Wishing Well. We made our own "Wheel of Wow" out of a wooden charger plate, dowel rods, and a napkin holder I found at a secondhand store. (All this for about $3.) We made a rainbow wheel out of paper to divide the sections, and tacked a 1-inch piece of mini-blinds (cut free at Lowe's just for asking

)onto the top as the "spinner". Then we labeled the pieces with prizes we had available to win. Most of these were trinkets I had saved from other places: 75% off Target "Dollar Spot" items, leftover party favors from previous parties, happy meal toys, and second-hand store grab bag toys.
For the wishing well, I tied a bucket onto a brick-look planter (another thrift store find!) and filled it with water. Then, using a ladle, the kids could try to scoop aquarium stones out. When they scooped one successfully, they could get a prize (for the boys, I had water guns, and for the girls I had Princess perfume-look water spray-bottles I'd gotten at the
Disney Store). They also got to keep the stone. They were shell-shaped turquoise stones and reminded my daughter of one of the blue gem hunt gems. I got these at Wal-Mart in the floral section.
3) Tournament Arena. We got a bunch of board games similar to ones the kids can play online, and set them on a table. Connect 4, Battleship, checkers, etc.
Here is a photo of some of the guests at the Tournament Arena:
4. Curio Shop. Arte's Gem Hunt is here, and this was everyone's favorite. We cut the "eggs" of a cardboard egg carton and hid the aquarium stones inside the egg cups. I taped them shut and my husband hid them in the grassy area around the pavilion while the kids were playing games. When they went on the "gem hunt", they got to search the park and pick a "rock" and if it held a gem, they got to keep it. Some held "slag" (wadded-up tissue paper), so they'd have to pick again. Besides the shell-shaped stones, I had also gotten a package of multi-colored ones that were round and cubed and other shapes. Each one represented a different Webkinz gem.
When everyone was done playing games, they came back to the "Clubhouse" for the birthday song/cake. While they were eating, I hung the pinata. I had a Princess heart-shaped one from her previous birthday, and I filled it with candy and more cheap trinkets, then covered the Disney princesses with a posterboard heart that I colored in rainbow stripes (to look like the Webkinz care heart). It was a pull-string pinata, so I re-rigged it to open again with new ribbons. When the kids were done eating, they pulled the pinata and got to fill their goody bags with the candy/trinkets.
The brownie cake, with #6-shaped candle:
In all, this party probably cost me under $100, which included the pavilion rental, decorations, refreshments, and prizes. We invited as many people as she wanted, since it was at a park, and a good amount turned out. (My guess is 25 kids, including all the siblings that tagged along.)
As far as I know, everyone had a blast, and I've still had parents compliment us on that party to this day.
Hope this helps....