How and what can I do to validate the customer journey/decisions when it comes to making travel sightseeing and activity purchases?
The same methods are used in validating assumptions and marketing strategy regardless of industry. You can find several discussions of this already posted on CFL and in many other places. The very first step is to make a list of ALL your assumptions. You will be testing each one of those individually. There is a massive amount of discussion out there about the travel industry, especially as it has changed over the last few decades. I'm sure you can find a lot of the information you need already described. But I encourage you to confirm that it's current by talking to real people too. Remember that it's gathering better information to ask people what they do or have done, than to ask what they would/might do.