Simple answer. Find happy path. First, understand the domain and nature of the business from the client. What they want the visitor to do on the website. Make sure that user could complete happy path smoothly.
Eg. For ecommerce website, happy path will be. User comes to the website, looks for an item using item name, the search result page displays, click on the iteam. Item is displayed. Add item to the basket and complete checkout process.
For your website, possible happy path can be, user click on tour item, checks availibility and go through the booking process.
To download test case excel example, you can use below link.
https://github.com/qamate/orangehrm-selenium-automation/blob/master/orangehrm-manual-testcases.xls
Feel free to write back if you have any questions.