A grid of pressure sensors would really be too much for me. Also because I'm thinking of something simple, just a gadget that I can take with me.
Is there a way to save these logs? Is it possible with just arduino, or I would need another peripheral?
If you wanted to detect random movement, and you wanted it simple and cheap, you could do a "pin-in-spring" detector sensor: Take a piece of small gauge bare wire, and wind a small coil, and put a small drop of solder on the end. Solder the other end to your PCB. In the center (soldered to the PCB as well) put another thicker gauge bare wire, that extends to the end of the coil.
At rest, the coil shouldn't touch the center wire, but when shaken a bit, the mass of the solder will cause the coil to shift and make contact with the center wire. Hook the two contacts (from the coil and the center wire) up to the Arduino via one of the pins which can detect an interrupt, and in your interrupt handler, set a variable to a non-zero value.
In the rest of your code (in a loop, most likely), have a counter or timer running; when the interrupt occurs, store the value of the timer/counter in the EEPROM memory, or better, to an SD card. Alternatively, you could dump it out the serial port and log it on a nearby PC.
If you stick with just the EEPROM or SD card (or micro SD and a Lilypad?), you could make the device really portable. You could do the same thing with an accelerometer, but it won't be as cheap, nor as easy to interface with.
If you did want to go the accelerometer route, though, you could probably make a nice "stack" of Lilypad boards; the Lilypad, then the accelerometer, then a micro SD interface...
