I built a few receiver tubes (think trailer hitch) into my steel benches that i can plug jigs, fixtures, tubing bender, etc into. These are very handy, i recommend them to everyone.
I have a couple 18x24x1" thick flat plates that i can plug into the receivers, which turned out to be my most used work surfaces. Clamp to 'em, weld to 'em, or beat the hell out of 'em, just hit them with a flat face 36 grit flap wheel for a few seconds and they look like new. I've got a cutting table, but i do 90% of my one-off fabrication stuff on these plates. Put a lot of thought into the height of your work surface, ergonomics can make a huge difference when you are trying to do nice work.
I have an engine stand head, tranny case holder, and diff housing stand that can plug into the bench's receiver tubes, as well as my assorted jigs and fixtures. Very solid setup, makes chores like setting pinion pre-load and dealing with crush sleeves a breeze. Under the bench is a rack with about 30 various clamps hanging from it, as well as an assortment of magnetic helpers to quickly fixture the odd stuff.