I usually carry 2 exta locks: a sturdy disc-lock, and a relatively light cable-lock. A bike cover is also a surprisingly effective way to ´hide´ it from view.
But none of these will help, if a professional thief decides to come and take it, and if there´s a gang of them and they have a van, it´ll be gone in seconds. So in my view the # 1 thing will be deciding the place, where you´ll keep it in the nighttime.

Any place, where there are some obstacles, so it´ll be impossible (or time-consuming) to get a van just next to the bike, will help. Being out of sight from the road, in the backyard, for example, will also. Thieves basically hate to spend time stealing vehicles, because there´s a possibility they could be caught while doing it. So if yours is put in a place, where it will likely take some time to get it out of, then it is more likely they go and find an easier target.

Fortunately hotel and guesthouse owners & staff in many parts of the world understand your worry, and wont mind, if you first ask politely, and then push your beloved bike to a safer place. In more expensive places, it is also common to have a guarded parking.

I could be wrong, but I believe that most of the thieves will not be able to deal with the (good-quality) disc lock in a very short time, so wont be able to push it or ride it away. Lifting the bike into a van remains my biggest concern - any cable lock can be cut in the blink of an eye, and even those that could possibly resist for a few seconds, weigh too much for me. This is why I never leave the bike on the street for the night, if I can avoid it.

When leaving the bike in the daytime, to go walking to some sights, etc., I just use the disc lock, or both extra locks, and then just hope for the best. If there´s a ticket booth, I´ll try to park next to it.

With the disc lock, it´ll also be important to always make sure that you have taken it off, before you continue your trip, or you will have a quick and embarrassing stop!