How fun! Not sure the age of the kids...But you definitely need to rent one of those big round yellow coolers from McDonald's to serve drinks in! LOL...they have those at EVERY construction site!
It would be really cute to serve chips, etc on the backs of dollar store or walmart plastic toy dump trucks. Brownies would be cute in chunks, served the same way...they would look like dirt!
Cute ideas here: www.birthdayinabox.com/party-ideas/guides.
I was going to post a pic of a construction cake I made (I am a cake decorator) but doesn't look like I can :(
Hope that helps!