Hi J.,
I'm a retired 1-2 teacher. I guess you have to decide who the invitation is meant to make an impression upon. If you want to get the enthusiasm "pumped up" with the invitees I would get inexpensive hot wheels or dollar store construction vehicles, make a simple invitation on a decorated notecard or computer paper, roll it and attach the vehicle to the invitation with colorful ribbon. Believe me, the kids will be thrilled and already have a small momento of the party beforehand. You could stick with trucks with shovels, etc. and also include a small candy favor . Birthday a In a Box online has great stuf to choose from. Also Oriental Trading Compay y might be worth checking out. I passed out many invitations for parents in my classroom and always noticed many didn't even get opened by the children they just asked the birthday child if it was for a party and then put it in their backpack. So, I'd decide on my audience and then design the invitation. I saw some great ideas for invitations for adults already listed. I hope your party is a huge success! Be sure to include some construction-type games int he party.
A race to bang nails into a board, screw nuts and bolts together, etc.
Take care,
L. in Arizona