I've recently downsized my camera to a Fujifilm X100s and I'm really happy with it so far. However the one thing it can't do, and that I'll miss from my 5DmkII, is the ability to do timelapse photography.
So I've built a little contraption with a servo that actuates a mechanical shutter release cable. It's run from an Arduino Nano and has a 7.4 volt LiPo RC battery. I 3D printed a structure that holds the servo and the shutter release cable.
It's pretty basic to operate, turn it on then press a button to choose how long between shots (each press adds 5 seconds so two presses would be 10 sec interval - a LED flashes to signify how long after each press), then press another button to start.
I haven't really tested it out yet so I don't know how long the battery will last but I think it should be plenty.
Here's a quick video of an earlier version made using an Arduino Uno and an early version of the 3D printed bracket thing:
https://www.youtube.com/watch?v=fM1CR5ofFA0