Build a 555 based adjustable timer. The 555 timer IC is a great little device.
http://blog.makezine.com/archive/2009/12/make_electronics_and_the_555_man.html
It can be used in many different applications.

In this Instructable, we use the 555 timer to create a countdown timer. The timer starts with a shake and ends when the LED stops glowing. There's no fancy accelerometer, no Arduino, no coding, just a 555 chip, some miscellanous discrete components and a tilt ball switch. The timer can be adjusted from one to about ten minutes. This particular timer was intended to be used between sets during weight lifting sessions. That is why it is only a one minute timer.

Fail
The final design is really version 1.5. The first version was designed to flip to start and flip to reset.
See the photos. The tilt ball switch proved to be too sensitive for a flip design. The circuit was redesigned to trigger on a shake. Hence the birth of the Shake Timer!

Inspiration/Sources:
A big thanks to these sites for the circuit design.
Adjustable 10 Minute Timer Project
http://www.kpsec.freeuk.com/projects/timer.htm
Bowdens Hobby Circuits
http://www.bowdenshobbycircuits.info/page9.htm#555mono.gif
555 Tutorial
http://williamson-labs.com/555-circuits.htm

Tools:
Drill
Saw
File
Sandpaper
Pliers
Cutters
Hot Glue Gun
Soldering Iron
Exacto Knife
Medium Sandpaper

Parts:
Proto Board - Radio Shack P/N: 276-148
555 IC - Radio Shack P/N: 276-1718
100K Resistor (times two) - Radio Shack P/N: 271-1347
470 LED Resistor - Radio Shack P/N: 271-1317
1Meg Trimmer Pot - similar to DigiKey P/N: 3309P-105-ND
8 Pin DIP Socket - Radio Shack P/N: 276-1995
220uF Capacitor - Radio Shack P/N: 272-1029
0.1uF Capacitor - Radio Shack P/N: 272-1053
Green LED - Radio Shack P/N: 276-022
1N4001 Diode - Radio Shack P/N: 276-1101
9V Battery Clip - Radio Shack P/N: 270-324
9V Battery Holder - Radio Shack P/N: 270-326
9V Battery
Tilt Ball Switch - adafruit
Micro Power Switch - Electronics Goldmine P/N: G16674
Misc wires
2.5 inch diameter mailing tube - recycle bin
Adhesive backed printer paper
Clear adhesive backed shelf paper or clear laminate

Step 1: Build the Circuit

Build the circuit according to the schematic. The schematic is very close to the component layout. When laying out the components make sure the board will fit on the inside of the mailing tube cap. Stuff the proto board and solder it up.

The photos show a modified version 1. A less ugly verion can be put to together when starting from scratch.

When done soldering everything, test the circuit. Temporarily attach the battery, power switch and LED. Shake it. The LED should light up for a period of time and then turn off. Adjust the length of time the LED stays on by adjusting the trimmer pot.

Step 2: Make the Case

Prep the tube. Just measure out 2.5 inch length of mailing tube and cut it off with a saw. Make sure the cut is straight and level. Sand off any excess.

Make the label. Print out the attached Label.pdf. Use adhesive backed paper. Full size mailing labels works great.

After printing, cut out the label. carefully align the label on the mailing tube to attach. Be extra careful because there is only one shot when attaching the label to the tube. Add clear shelf paper or laminate over the top of the label. The labels will last longer this way. Trim off the excess (the label was purposely made oversized) with an Exacto knife. Drill holes for the power switch and LED.

For the labels for the caps, add the clear shelf paper first then cut and fit to size. Peel off the backings and attach them to the caps.

Label.pdf
Download

Step 3: Install the Components

Cut about 4 inch long leads for the power switch and LED. Solder these to the board and the components. Solder the 9V battery clip to the board.

Fire up the glue gun. Glue the board to the TOP mailing tube cap (it should be labeled Shake Timer). Glue the battery clip to the BOTTOM mailing tube cap (it should be labeled Battery). Note: It is probably better to mount the battery on its side, not on the edge like the photo shows. Play around with the location of the battery and use what is best. There is not much room inside.

Install the switch. Install the LED and add a dab of hot glue on the backside to keep it in place.