I don't think you can solve this with just APIs currently. There's a ton of information currently but extracting the useful bits and creating a complete trip plan just isn't possible today.

Heck I've done the equivalent of screen scraping by visiting tripadvisor and lonely planet thorntree forums and even that was not enough. I tried a new site called triptips.com and found they had good balance of human intervention from locals to simplify the hardest part of travel planning.

Its relatively easy to book a flight or hotel or air bnb. The hard part I've always found is determining which hotel makes sense given what i want to do at the destination. should i rent a car and so on..

I've used a service which is a bit like TaskRabbit for Travel to plan a trip to Hawaii - you can see it here https://www.triptips.com/#!/place/maui-hawaii

It's more about the "how" than the "what". If you have a general idea about what you want to do the trip planners give you unbiased advice on how to do it and save you maybe 3 or 4 weekends of trip planning

That's interesting, but it's not what I was talking about and doesn't solve the OP's problem of having to reacquire users every nine months.