If your friends are taking an overnight trip and have asked you to dog-sit for them, the tips here could come in useful.
Get the name and address of a nearby after-hours animal services clinic
If their dog is healthy and they're not the type of people to worry about emergencies, then your friends might not feel the need to look for and pass on information to you about their local vets who provide after-hours animal care services. However, even if you don't expect anything to wrong whilst you're dog-sitting, it would still be sensible to obtain this information yourself before your dog-sitting work begins.
The reason for this is that even if the dog has never had any health problems, it's possible that they could either get injured or suddenly develop a health problem whilst they're under your care. If this happens, you'll have to look for the clinics that offer after-hours care service and decide which of them looks the most reputable while you're distressed and the dog is suffering from what could be a time-sensitive injury or condition. Getting this information before you dog-sit and ensuring that you also have a copy of the directions to the vet clinic you've settled on will mean you can serve as a responsible dog-sitter and can deal with any pet-related emergencies that are thrown at you during your stay.
Don't be too adventurous if you don't know the area or the dog that well
If you don't live in your friends' local area, it's best not to get too adventurous and try to explore it with their dog when you take them out for their walks. The reason for this is as follows; if for example, you let them off the lead in an area that you're unfamiliar with, and they run off, it could take you ages to find them and you could get lost whilst doing so. In the meantime, they could end up eating some toxic wild berries or getting bitten by a wild animal. In either case, you would end up having to avail yourself of the aforementioned after-hours vet service.
Likewise, if you have not spent much time with this dog before and don't, therefore, know their temperament, and you take them for a walk around a jam-packed park, there is a chance that they might behave badly in a way that you cannot handle. For example, they might jump on sunbathers or chase people's frisbees and run away with them. As such, it's best to opt for quiet and familiar walking routes.