If you go on playhousedisney.com and nickjr.com (I believe nickjr, not sure b/c we don't watch it all that much.) you could print things out and use them at the party. They also have party idea links on their sites. on the pbs kids website they have pin the bow tie on Arthur (all free printables), invites, thank yous, happy birthday banners, etc. I also made an arthur cake to go w/ that one.
DD was diagnosed with a peanut allergy so now I make ALL kid birthday cakes for the family since she can't have any bakery items. If you google your theme I am sure you will be able to find some nice ideas. I made a cheap Lilo and Stitch cake for DD's 3rd birthday and we has a Hawaiian Lua theme in January
hee hee. Also, the little toppers that go on cakes, if you go to your grocery store and ask to buy them, they will sell them to for like $4-$7 depending on the topper. Also, Wilton cakes decorating things are usually $1.99. I also did a cool spiderman cake that was super easy (time consuming, but easy and one of the cheaper cakes I made).
Hmm...what else...my dollar store sells balloons and all you really need is a few.
If you are planning on doing an dinner then cake, pasta is always cheap. For one of DD's birthday I made Giada DeLaurentis (the girl on the food network w/ the big head and little body) Lasagna, it WAS AWESOME and pretty cheap. Throw in a salad and done
We also did a cold cut platter one year and just had cold cuts and potato and macaroni salad and some cole slaw, it wasn't that expensive, but the lasagna was way cheaper. I just did not feel like cooking b/c I think I had finals or something crazy going on.
And just take A LOT of pics...