Miniature chainsaw, mortar and ping pong blower
(a few tiny ideas)
Basically a toy chainsaw that can cut through paper. Using hobby-R/C-like parts such as those tiny light motors would be a start. Reduced size, like a cellphone at most, mountable on a figurine soldier would be great.
The miniature mortar should throw marbles on "mortar trajectories", you should be able to set the angle of the launch and (optionally) somehow the speed, though this wouldn't be necessary if the angle part is done well and the power is relatively constant at all shots. For ease it should be built only for angles above 45 degrees and the angle slider (or whatever the solution) should have marks (ticks) such that a user can remember and even create tables if he so wishes, in his war against coffee cups. Simplest solutions of "powering" would be rubber bands or coils from broken toys.
A ping pong blower would be a tiny but strong fan (brushless outrunner motor) that keeps a ping pong ball "levitating" in mid-air straight up, but most excitingly even blown under an angle too. This wouldn't be a problem (physics takes care of it alone) just that the mount of the fan (ducted or not) should be operated and tilted as needed. This "levitron" would be a nice display on the desk running as long there's juice in the battery or else.
Sorry if I'm not posting in the right place, these are just some ideas i'd love to build but i'm too clumsy and lazy :(
Though my source of inspiration is the variety of USB gadgets commercially available, i'd like these to be more "free" and simpler if possible.