We've been going through all kinds of food based building (extremely geometric!) structures. The other thing we do when he builds with toothpicks and gumpdrops/marshmallows/gummy bears, etc. is he likes to stack cards on them, and then items to see how weight bearing his structures are.
He recently finished up a science class, and the last class was building load bearing structures using Zoobs. The kids built structures using Zoobs, and then when they were done the instructor had them put their structures on a scale, and he stacked books on them until they either broke, or the books were unable to be stacked higher due to instability. DS was so hesitant to put his structure on the scale because it wasn't perfect, and so I offered help (thankfully he accepted it!) and after class the instructor let him put books on his structure. He was so incredibly excited because he beat the class record with 119 pounds of books! We would have kept piling on the books, but the pile was very unstable because it was over 5' tall.
We also have a hydrodynamic kit, which is very cool! A huge and complex marble maze set, K'Nex (we bought a huge mixed set off of Craigslist), Erector Set, and we just bought this really cool cardboard building kit from Klutz
http://www.klutz.coresense.com/Buil...-Ships?merch_location=building%20Listing