There's a great website called Free Printable Invitations where you can download cute and colorful invitations. You can also choose to download an editable version of any invitation for a small fee.
My little guy just started kindergarten this week (sniff, sniff) and I will probably use one of the invitations on the site when it's his birthday for him to give to his classmates. I really like the van invitation pictured above but it'll be up to J to make the final decision as to what he wants.
Free Printables also has a newsletter signup on the site where you can receive the latest news and designs. Have fun!