A combo thermometer and hygrometer is a useful tool to have for hotels.
My favorite use for one of those is to use the shower as a makeshift humidifier. Most hotels don’t have windows that can open for a variety of reasons, such as the building being high up, folks smoking in the room when there is a window, or simply that it costs more to install them.
Combined with an air conditioner, the air in your room can get very dry, on the order of 20-30% relative humidity. I’ve found that running the shower until you see about 55% humidity in the bathroom translates into a reasonable humidity level overnight. You don’t want it to be too high or the room will smell stuffy, or more severe cases, cause condensation, making it very uncomfortable to sleep. With a hygrometer, you can be careful not to go overboard.
Another reason to get one of these (and really, the original reason that I bought this) is because sometimes the thermostat in hotels will lie to you. I’ve been in rooms (looking at you Vegas) that are actually 75ºF (24ºC) when it’s supposedly 70ºF (21ºC). If you don’t have an independent way to measure the temperature, it can be difficult to distinguish if you’re warm because you, for example just walked in the heat, or if you’re warm because it’s actually warm.
If you find that your thermostat is lying, sometimes you can call down to the front desk to have them fix it or if it’s made by Inncom, you might be able to adjust it yourself: override hotel thermostat controls.
It’s also useful for adjusting minifridge temperatures, which I talk about in the linked post.
The above instrument is this cheap one on Aliexpress for about $6. I ended up using gaffer tape to keep the battery compartment from opening, so it’s not necessarily the best construction, but it is in a compact size. It is however, cheap, so it’s not a huge deal if you lose it and have to buy another.