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..