Backyard BBQ's are always a great time! For centerpieces I would check out The Front Door. The table accents section offers favor ideas and centerpiece containers. Also check out the ideas section. You might be able to find something you like there.

Not sure if you have picked your invites, but I would also take a look at the print your own section. They have some BBQ and party themed invites or if you are looking for something more personalized try the photo invites.

Hope that helps! Have fun planning.