Unfortunately, there are not a lot of options out there. Expedia and TripAdvisor (Owned by Expedia), along with pretty much every other major travel website I’ve looked into, will not offer API keys for their travel data unless you can provide justification that you are a credible application developer who can drive business to their website.
The US Government has a wealth of APIs available to the general public, and although I don’t think you can get flight availability, hotel reservations, etc., you might find some of these data sets useful for a road trip. Walkability Indices, Road Conditions, Traffic and Weather Cameras, Travel Warnings, and the list goes on: https://catalog.data.gov/dataset?q=travel
Sounds like a fun project, good luck!