Dinosaurs were the theme. (We included a few dragons, too, since I'm a bit of a crypozoologist and feel the legends of dragons are probably based on dinosaurs.) I began with a simple invitation; I've found that keeping it digital makes the invitation hassle- and cost-free. In this case, I found an invite I really liked, then created something similar for our own purposes. (Cost: $0)
Next, I considered decorations. I wanted to absolutely minimize costs in this area, since most decorations are used for the day, then thrown away. This year, I didn't buy banners, helium balloons, or even streamers.
I didn't mind buying some plain balloons. (Cost: $2) I thought about sticking to green and orange or yellow - which seemed dinosaur-like colors - but instead I used a rainbow of colors. I hung these everywhere I could think of, and they did a great deal to create a festive look.
3-D dinosaurs (including a "Pterodactyl," Triceratops, T-Rex, and "Brontosaurus") and printed the Pterodactyl (actually a pteranodon) on my home printer, using cheapo ink cartridges. (Cost: difficult to estimate, but definitely under $2; if you don't have a printer, or your printer ink is expensive, places like Staples print inexpensively.) I hung these with thread (Cost: I already had the thread and used just pennies of it. You could also use ribbon or string you have on hand.) from the ceiling fan that is over the buffet table, plus in a few other places. My 3 year old thought these were magical, and the birthday girl couldn't wait to add them as decor in her room.
In years past, I've always made something fun for the other kids at the party (all cousins) to take home. This year, I did something simple. My daughter and I made dinosaur treat bags from brown lunch bags and filled them with dinosaur fossil footprints cookies (full instructions here). (Cost: We already had the bags, so it cost us pennies; if you have to buy bags, expect to pay up to $1 or $2 for more than you need; the cookies were also made with ingredients I already had, but I'd estimate the cost at $2, tops.)
I also found some free printable dinosaur masks. I was going to color them in on the computer, but ran out of time. So I just stapled elastic (each end of it knotted) to the masks and let the kids take the masks home to color. (Cost: about $2)
We kept the food very basic (Cost: about $70 for burgers, hot dogs, salads, and chips), but I've seen other parties where they named all the food in a cute way. For example, "T-Rex 'Taters" and "Brontosaurus Bananas." I thought about doing more general categories (marked with cute signs), like "Carnivore," "Herbivore," and "Sweetavore," but I just didn't have the space on my table for this.
For the cake, my daughter had very specific ideas. She wanted a smoking volcano cake - with a T-rex eating a cow off to one side. I mostly followed the instructions here, but, trying to use only pans I already owned, chose to bake the bottom part of the cake in a bundt pan, the middle part in an 8 in. round pan (I trimmed this cake a bit to shape the volcano), and the top in my 4 cups Pyrex measuring cup.
Finally, there were the games. We played "hot dinosaur egg" (which is just "hot potato" with a plastic Easter egg in place of the potato), while playing a dinosaur song I found online. We also did a dinosaur egg relay where I gave each child a spoon and placed a plastic Easter egg on it and had them walk a course as quickly as they could. If an egg fell from a spoon, that player had to start over at the beginning. It proved fun and challenging for all. But the most popular game was probably "dinosaur egg stomp." I tied a balloon to each child's ankle and told the kids to defend their "egg" while trying to stomp on and pop everyone else's. We played that a few times. (Cost: I had everything needed for these games, so our cost was $0. But you might need to buy a bag of balloons and some curling ribbon at the Dollar Tree for $1 each. It's best to buy the plastic eggs at the Dollar Tree near Easter time, $1, but if you need them out of season, you can buy them at Oriental Trading for $4 plus shipping)
Other ideas I didn't use, but liked:
* Using ferns and rocks as decoration.
* Dino soap favors. I was going to use blown out eggs, ready to pour glycerine soap, and tiny dinosaur toys to make these, but not having any of these supplies on hand, I ended up taking a pass. Still, these soaps are a fun way to get kids to wash themselves!
* Edible dinosaur bones.
* My daughter loves party hats, and these dino hats would be very easy to make. Just form a cone from cardstock, punch holes for ribbon ties, and add the saw-tooth decoration.
* Free printable dinosaur pennant at NickJr's website.
* Dinosaur rocks, finger puppets, dino feet, dino tails, and many more ideas here.
Total Cost of Party: about $99.