A humorous invitation can help set the tone for this group effort. Make it clear that the guests are coming to work. You don't want someone showing up in a dress or suit.
There's very little need for anything more than a simple centerpiece.
The guests are giving their skills and elbow grease. You may consider giving tools to the couple, or chipping in for materials.
Games and Activities
Painting, hammering, hauling, gardening, sawing, shoveling, and fixing should be enough!
These people will be hungry after working a while. You can either make it a potluck or provide them with the food. Hearty sandwiches, fruits and vegetables, cole slaw, wraps, coffee, soda. Their final task of the day can be to create their own do-it-yourself sundaes. Or you can just order pizza.
Have someone take instant photos of people working throughout the day. Punch a hole in the bottom corner of each picture and, with a ribbon, tie the photo to a bar of soap. On the back of the picture, in permanent marker, write the date, the couple's names, and a big "Thank you!"