The first step in building the playground was to construct the main square "fort," using 4x4" pressure-treated (PT) lumber for the 4 vertical posts. There are 2x6 PTs horizontally around the bottom for stability, and 2x4s around the middle and near the top. The plans call for building a sandbox inside the bottom of the fort, which could easily be added. I opted not to include it. The lumber I used was very heavy, but I was able to do most of this part on my own with the lumber on the ground (my kids helped me hold it up when it was time to connect the sides together).
Here you can see one side of the fort assembled on the ground.To connect the lumber, I used 5/16" x 5" galvanized steel carriage bolts, washers and nuts, because they will not rust or react with the pressure treated lumber outdoors. I ordered them in bulk quantity (60 of each) online from www.wholesalebolts.com, because they were much more reasonably priced (about $25 vs. $80) than in the local stores.
I also had to buy a new corded drill, because my cordless one just didn't have the juice or torque for all of that drilling. A ratcheting socket wrench with a deep-socket attachment was also very helpful in tightening all of those nuts and bolts. And of course, I used a carpenter's level to make sure everything was lined up just right. Seeing it all come together was very exciting!