On engineered gravel roads and other mild off-road jaunts, I ride 30mm tires on a road bike. As far as I know, Kenda Kwick are the knobbiest 30mm tire available, and it seems like the folding bead ones might even have been discontinued, although some are still available.
On tougher gravel courses that might only get a blade drawn over them once every 10 years or so, I've been known to just get out there on a hardtail or full rigid MTB.
Some trips are mostly on smoooth double track with a couple of rock gardens and a sandy arroyo crossing or two, and I'll break out the fatbike with plus tires on it, I save 4" tires for snow use only.
