My son's favorite birthday party was a Shrek themed party. We had a "Worlds Worst (best) Ogre Competition" The invitations announced to the invited Ogre that he/she was invited to a celebration to find out who was the grossest, messiest, and scariest Ogre of them all.
I suggested the kids wear old ratty clothes since we would be getting messy.
We had green face paint to turn all the kids the appropriate shade. They had a bugger eating contest which was comprised of green jello in small cups they had to slurp up with a straw. We had a Swamp Dunk competition. They went dunking for apples in a water that had been colored brown with a bit of oatmeal floating around in it.
There was a belching contest where the kids all got to drink a root beer and then see who could belch the longest, loudest, say the most letters of the alphabet in burp mode.
In the farting contest they competed for who did the best, loudest, and most realistic armpit fart noises. Those kids were so funny I about died laughing during this part of the party.
The Swamp Rat Fling was a huge success, we had a few rubber/plastic rats purchased at a 90% off sale after Halloween that they threw as far as they could. My hubby built a small "rat-a-polt" inspired by a local ren fair event.
All of the food on the buffet table had funny names. I had a bowl of peeled grapes (wombat spleens) , spaghetti with meatballs (also known as guts and gizzards) and the veggie tray (corpses of snarky plants).
The cake was a choclate cake cubed up trifle style with choclate pudding and gummy worms with a gummy rat on top.
We used the fake spider webbing from the aforementioned Halloween sale. We covered a few things in burlap.
We did an Ogre Swamp Magic show. I had grated up a whole head of red cabbage and stewed it with water to extract the natural purple dye of the cabbage, which just happens to be a wonderful pH identifyer. I then strained out the pieces of cabbage and stored the juice in a clear pitcher in the garage to contain the stink.
On the day of the party we did this demonstration outside, I wore goggles and chemical gloves. Before the test begain I carried around the pitcher of swamp juice for the kids to smell. It was disgusting and the kids LOVED it. Each of the test materials was already in clear mason jars and then I poured on the "Swamp" juice (the red cabbage juice) and watched the colors shift and change. It is very quick and quite dramatic.
Here is a good link to learn more, this is in the sense of a science experiment but it is not difficult to change this up for a party setting.
http://chemistry.about.com/od/acidsbase1/a/red-cabbage-ph-indicator.htm
In all it was a fantastic day, very inexpensive compared to some party supplies, and unique. At the end each child got a sash (More burlap decorated with a glue/glitter award) So, there was a Messiest Ogre, Loudest Ogre, Stinkiest Ogre, Silliet Ogre, Strongest Ogre, Slimiest Ogre, Scariest Ogre, Most Gluttenous Ogre, Belchiest Ogre, Fart-ing-est Ogre and then there was Birthday Ogre. I made as many sashes as kids who RSVP'd and then a couple more for "just in case."
I did not purchase any actual "Shrek" merchandise, but just used cheap green supplies, dug out old dirty stuff from the garage and barn, and then Shrek inspired activities. Generally I just let my son revel in gross stuff for an entire day, he was thrilled.
