Some tours aren't affected by rain and others get even more exciting. However if it is a very heavy rain and the attraction judges necessary cancel the activities for safety issues, you can change the day (if available) or you will get a refund from the agency itself. We also recommend that you bring a raincoat and umbrella so you don't have to miss anything from your tour and your fun.
You can also prioritize on rainy days to visit closed parks and museums. There are many options e usually all of these attractions remain open in case of rain.