Cable lock with a relatively long cable, long enough to go around a pretty good-sized tree. We've left tent, gear, bike, and laundry drying in campgrounds all day, but we take our valuables with us. At restaurants, sometimes we lock, sometimes not, depends. At grocery stores or other shopping locations, someone always stays with the bike. If I were solo, I'd bring the bike in. In lodgings, we either bring the bike to the room with us, or secure it in a provided locked room for bikes. It's good to tour on a tandem.
The cable lock probably weighs a pound. Angle grinder opens all locks. It's fun to watch the video of the guy in NYC unlocking his bike with an angle grinder in several busy locations.