Roller Coaster
The long and narrow building pieces of K'nex lend themselves well toward building a large roller coaster. Standard pieces will work fine for the most part, but to create the tracks, a builder will need the plastic tubing that is not common with every model package.
Life Size Robot
With a little imagination and a lot of persistence, it is possible to create a 5 to 6 foot robot out of K'nex. Be sure to keep balance and build strength in mind throughout the process, as this will most likely be a tall and narrow structure.
Ferris Wheel
Different sizes of Ferris wheels can be easily built with K'nex. In fact, can start with a small wheel and, by adding additional pieces, it is quite simple to expand the wheel to a much bigger size. The only additional feature needed is a base capable of holding the wheel upright. To add more interactivity, add a common electric motor to make the wheel spin.
Catapult
Because K'nex pieces are long and fairly sturdy, it is possible to design and build a functional catapult that uses rubber bands to launch projectiles. Be careful not to put too much stress on the plastic pieces with a heavy rubber band because they may snap and cause injury.