On the path to where ever the heck I am going I learned about being an affiliate about 20 years ago and it was an "job opportunity" to sell steak knives.
I passed but, it was a new dawn and you too could sell tupperware on the Internet ! Well, to say the least I am not an affiliate but I am reconsidering now ...
Below is a few good travel affiliate programs just remember some require a lot more time and money for all your efforts.
Trip Advisor is one of the better ones but remember again you have to dedicate a lot of time and effort into the promotion too.
STA Travel has the same program as Trip Advisor thru the same network. So to recap, already you join one affiliate market then the actual affiliation opportunity.
BCD Travel again American based and doesn't let you in the secrets till you can get going ...
Booking.com has integration tools to work with too.
FlightNetwork is good too just a lot of promotions
All these (and more) affiliate programs can be found on networks which means its a Big Craigslist place and you have to bring something to the table.