I didn't find a guide or much on the internet about this topic so thought I would try here:

How would I set up two identical Mikrotik boxes to make them redundant?

I have a few interfaces, one is bridged and another NATed.
Is there a way to assign the SAME IP to two MT boxes as the gateway for the NAT? That way if I need to take one offline, the other just takes over.

I would like it to be active-active where they are both doing stuff all the time in maybe a load-balancing fashion. But if that isn't possible I'll take a hot standby config or active-passive with some sort of auto fail-over.

At the very least I would take a method for starting up a spare redundant MT box with the exact config as the first in case of hardware failure.

Anyone have any suggestions on this?

Take a look at VRRP protocol (Virtual Router Redundancy Protocol) - this is likely what you want for standby solution:
http://www.mikrotik.com/docs2.8/ip/vrrp.main

A load-balancing solution is not possible the way you describe it - you can give two active boxes on the same segment identical addresses...

Best regards,
Christian Meis

Awesome, thanks!

Now what is the best way to make my bridging redundant?

What would happen if I take two MT boxes, both bridging the same network segment? Does this create some sort of bridge loop if there is just a non-managed switch on one side of the bridge?

If you are able to duplicate your physical layer, OSPF works well to load balance and provide automatic fail-over. We use this on our wireless repeaters to ensure redundancy.