A moving slimeblock Mech that is easy, small and maybe even buildable in survival.
Mech Mini 1.0 has a crappy engine so use 1.1 or 2.0
Mech Mini 1.1 has 2 legs that move forward every 2 blocks and get left behind when damaged, one tnt missile which can be fired by breaking a block of tnt, a 2 shot tnt cannon fireing every block (cubehamsters moving tntcannons fire once every 2 blocks).
The machine can also be started and stopped by updating a piston with flint and steel.
Mech Mini 2.0 is like 1.1 but it can be started by breaking a furnace and stoped by placing one. It also is cheaper then 1.1 .

If you are willing to make this on a server in survival please put that in the comments.