My 3 year old loves Diego. Now I realize that 6 and 3 are worlds apart in ages... but a jungle theme can be really fun. At our party, 1/2 were 3 year olds, 1/2 were 5 year old siblings. To decorate, pick a room (consider leaving it up for a few days because it will no doubt look really cool and the kids will enjoy it) cut paper bags in long strips, twist them and tape them together with brown packing tape. Hang these all over the walls in a vine fashion (let them drape down in the door ways a little and wind them up the sides of the door jams. Then cut a bunch of leaves out of green construction paper (just buy a pack of green so you have enough) and tape those all over the place. We put them on the vines mostly but then put a few here and there to fill in around the room. One game we had was find the tree frogs. We had a bunch of little tiny frogs that we hid under leaves and behind vines. Once we found them all, we put a big blue plastic table cloth on the floor with tape and taped lilly pads to it (green construction paper). The pads had points written on them. The kids threw the frogs on the lilly pads to see who could get the best score. For your crowd, use the entire length of the table cloth and put some smaller pads far away so they are harder to hit.
Another activity we had was a scavenger hunt. The kids got animal beanie babies to take home (lucky our neighbor had a huge number of them that they wanted to get rid of... couldn't get to sell at yard sales, so they were free). I wrote a scavenger hunt type clue for them and hid them around the house. The kids had to figure out the clues (easier for the younger, harder for the older). Make them figure out what they are looking for and then figure out where it is. If you don't have access to inexpensive stuffed animals, go to the dollar store and look for a package of plastic jungle animals.
Another activity was an obstacle course. We had blue painters tape laid out on the floor to indicate water and brown tape in the shape of rocks. The kids had to jump over the water, hop from rock to rock, crawl through a tube, hop back across the water and crawl through a cave (chairs set up back to back with space between, sheet draped over.
The kids had a blast doing it and then we told them to get in a line and do it as fast as they could, round and round. They were crazy and laughing constantly.
Another thing you can do is have them do pillow case races... potato sack races done with pillow cases. Also, use the pillow case to create 3 legged races.
You can fill a back pack full of all kinds of survival equipment... let the kids see it for 1 minute then put it away and have them write down (or yell out) what they remember seeing.
Another idea is to get out some survival gear and have the kids dress up in it. Get out some large jeans, a big hat, a big shirt, some boots, etc (adult size) and make them get dressed up in them. Make it a race and make two teams. They have to put all the items on then do some activity (jump up and down 5 times or do 5 jumping jacks, something that can be counted out loud) and then disrobe and tag the next team mate.
You can play the whisper game and have them all in a row. Whisper something like "slimy snake" into one kids ear and tell him to quietly pass it on. See what the last kid thinks was said, then rotate and put the first person at the end.
You can have fun food too, grapes are snake eggs, raisins or craisins are bugs, potato chips are beetle wings, hot dogs (cut into long 1/4 strips and then boiled so they curl up) are worms. Decorate the table with snakes and spiders (get some now since it's Halloween) and other creepy things. I saw a great web site with ideas like this when I was putting on the party. Search for Diego theme party ideas and see what you can find.
Another thing they suggested was to give the kids their goodies in brown paper lunch bags. Print out a label (cut it out of regular paper) that says something like "Andy's Supplies" and tape it to the front. You can find cool things to add to the goodie bags like magnifying glasses at Party City. I think I got a bag of 12 for something like $4.
I'd love to give you more ideas. We really had a blast with this theme and I'm even suggesting we find a way to do this for our Vacation Bible School in 2009 because it is so versitile and fun. If you decide to go with the jungle theme, feel free to contact me and I'll help you think of some more. Consider, the obstacle race and sack races can be indoor or outdoor so that is helpful if you have bad weather.
Good luck
Liz