What is an offer? It's simply the product you want to sell created into a package that entices your audience wants.
Offers don't have to be paid - they just have to drive your audience to take action
No one 'wants' to join your email list, but they will opt in for something great, such as a top ten list, or the three best (you fill in the blanks - could be hidden secret gem, could be river cruise itinerary - what will speak the most to your audience). When you create an offer that is irresistible because it's something your audience actually wants, it becomes an easy sell.
Many travel advisors think that 'they' are what makes it irresistible, but unless you are also the answer to solve the problems your clients are facing, once again that offer will fall flat. People don't want to travel with their "travel agent" - they want to travel with a friend, with an expert, with a guide.
In order to get your offer to be irresistible you need to first understand who you are creating the offer for. Then you need to figure out what are the problems that audience is facing and what is the best solution for those offers?
If you don't know what pain your audience is trying to avoid, you won't be able to fully understand what will motivate them to take action on your offer (paid or otherwise).
With every offer you put out, the question that needs to be answered is "So What". Your audience will always want to know 'so what's in it for them' - if you can't pass the so what test, your offer will continue to fall flat.