Many countries, especially in tropical regions, have requirements or recommendations for vaccinations like yellow fever and won't let you enter without proof you've had them. Use IAMAT to check.
Many cities have walk in clinics for these types of shots, but for those that don't it may take some time to schedule which is why we listed this for two months out. Don't forget to check whether your health insurance covers any of the shots, as some of them can be quite pricey.
Often the best local experiences are limited in the amount of people they can take. For example, restaurants on the World's 50 Best list and permits to hike the Inca Trail to Machu Picchu are usually booked multiple months in advance. Often this is for a good reason too: allowing too many people can quickly lead to problems with sustainability and maintenance. If these type of experiences are high on your list, it's best to be proactive and book ahead of time.