Proximity sensor (bluetooth or something) + thermite on the disks.
If you get further away from the machine than 5m or so when it's powered up it will melt everything. For extra kicks also wire up another detonator with a case ground wire or something, if the case is opened - it melts.
Flush crypto keys, activate screensaver, kill all active terminal sessions.
If you want to get extra special: scramble the disk keys. Without those, data's good as toast. And that can happen a lot faster than a shred, wipe, or even dd if=/dev/zero of=/dev/sda bs=1g
If you get further away from the machine than 5m or so when it's powered up it will melt everything. For extra kicks also wire up another detonator with a case ground wire or something, if the case is opened - it melts.