Should you stay at places where the security may be less than what you would wish, an inexpensive wedge-type rubber door stop, jammed solidly under the door at night (as far from the door hinges as possible), will provide you with a good measure of additional protection against unexpected visitors. Doors are hung at various heights, so don’t get one too thin. That said, rough/uneven floor surfaces (especially some types of carpet) reduce the efficiency of rubber wedges, possibly enabling a sufficiently motivated miscreant to force the door open anyway (although you are unlikely to sleep through the event). For more serious security, consider a Veritas® Traveller’s Doorstop; this hefty (7.1 oz, 201g) chunk of solid aluminum features a hardened steel anti-slip screw adjustment to tighten the wedge against the door, and prevent slippage across the floor. A door thus secured will break long before it opens (someone trying hard enough might damage the carpet a bit, but better that than you). And if you’re looking for a dual use in order to justify packing this, realize that you could probably fight your way out of a bar with one of these things!
Just remember that the use of any door security device will delay emergency exits from (or entrances into) the room.