We used 2 x 1 inch tubing with 1/8 inch walls for our chassis and lift tower and 1/16 inch wall 2 x 1 and 1 x 1 tubing for the other structures. Most of it was riveted using 3/16 inch rivets and 1/8 inch thick gusset plates that we made. Some joints, like the lift tower to chassis were done using 1/4-20 bolts to allow the lift tower to be removed for shipping. This construction was adequately strong for this years game and would probably be good enough for last year's game. Last year's robot was made from aluminum channel held together with eight 10-32 screws in the corners. There were fewer than the rivets we used this year and it held up fine with the abuse it was subjected to.
I would strongly suggest using washers under the heads of any socket cap bolts (see link below). They bearing surface area of the socket cap bolts is smaller than the bearing surface area of hex head bolts. I have seen quite a few instances where the head of the socket cap bolts have "crushed" the aluminum sheet due to the high concentration of force. Some joints have gotten loose because of the deformation of the aluminum. This is different from crushing a tube.
http://www.andymark.com/product-p/am-1207.htm
Do be careful where you use the 1/8 inch wall tubing. We ended up overweight and had to replace quite a few pieces of our structure with 1/16 inch wall tube to get the weight down.