If they’re backpacking you can get a metal security mesh that encloses the backpack to stop it being broken into when it’s out of sight on buses, planes, etc. But it’s a bit pricey. At the cheaper end of the scale (and I always take these with me): A length of chain and a padlock are great for securing a backpack or suitcase around a lamppost etc. in crowded areas. If the traveller is female a personal alarm wouldn’t go amiss. (Or a whistle). A good torch is essential as is a Swiss army knife. The cheapest and most practical is a door wedge to prevent the door being pushed open in cheesy 3rd world hotels.