Single Set Challenge: 30243 Umbaran MHC Polybag

You could almost consider this as shooting fish in a barrel.  The Umbaran MHC polybag is essentially tailor-made as a set for MFZ.  That being said, you can still build a lot more with it than the one mech that you are provided instructions for.

Ground Attack Vehicle.  Flickr page.

These two can be built at the same time.

3x3 Mech.  Flickr page.

Longneck.  Flickr page.

These two can be built at the same time.

Weather Station.  Flickr page.

Scorpio mech.  Flickr page.

Finally we have the Centaur Warrior.  Flickr page.  This one uses all the pieces from the polybag.  Stats for MFZ: 2Rh(staff)/1B(shield)/1G+d8(extra legs)/2W.

All of these designs can be considered for MittenNinja's 3T Challenge as well.