Like the LiveCD, but USB-stick based so you can write to it.
This is already implemented, easy instructions at https://wiki.ubuntu.com/LiveUsbPendrivePersistent
+ Easy: download, burn, reboot, play.
+ Easy to regret: just don't boot from the USB stick any more.
+ Complete: persistent data (configuration, user documents) is easy to handle.
+ Fast: loading from the my USB drive is less than 10 seconds longer than from my HDD, and 30 seconds shorter than from my identical LiveCD.
- Inconvenient: I have to reboot.