I try to not overburden myself these days for personal trips, but I do recall our trip to Austria a number of years ago in which I didn't adhere to that philosophy. I brought my Contaflex TLR with its 50mm lens and then I picked up an 85mm f/2 Sonnar lens for it from the LeicaShop in Vienna that I'd ordered. So that was like adding a lead weight to an already weighty camera bag. The other thing I brought on that trip was a Hasselblad SWC - it was actually a pretty cool camera to bring along, but clearly I overdid it with physical burdens. To be sure it was a wacky combination of cameras.
For commercial work I pretty much bring a carload of stuff -- the furthest I ever drove for a job was from where we lived near Baltimore, MD to Port Angeles, WA. I brought three Nikon digital camera bodies, I think four lenses, SB800 flash with a Quantum battery pack, Pocket Wizards, my very sizable Dynalite strobe kit with three packs and a number of heads and cables, many light stands and related light modifiers, sandbags (very important!), a large Manfrotto tripod, a smaller Manfrotto tripod, ladder, and I even managed to bring along a Leica Monochrom kit for personal work along the way. Pretty sure the only thing I didn't use on that multi-day job was my SB800 with the battery pack and one of the backup Nikon bodies.