Use the reliable www.booking.com
to find local hotels/b&bs/apartments etc. It has very extensive listings in all European countries, with many hotels etc offering free cancellation up to 24 hours beforehand. Most importantly, reviews can only
be posted by people who have booked through the site and completed their stay. I've used booking.com in many countries for 10+ years and have never encountered any problems: everywhere I've stayed has been just as I expected from the site info and reviews.
>Please also advise border crossings considerations.
are, along with 23 other European countries, in the Schengen Zone.
There are no
regular border checks between those countries. As a US citizen you can stay visa-free in the Schengen Zone for up to 90 days in any 180-day period. Date of entry and exit from the zone (whether by land, sea or air) are both recorded on a shared system so all countries know about any overstay.>low cost rental car options
There are plenty of reliable car rental options in all 3 countries, including multinationals such as Hertz and Avis, but
your options will be greatly reduced by wanting a one-way rental including drop-off in another country. Not all rental companies allow one-way trips and even fewer allow drop-offs in another country, for obvious reasons. You will also pay a potentially hefty premium for that sort of trip (to cover e.g. legally-required insurances).
You need to contact car rental providers in Porto
well before your trip to check if they will allow a one-way rental with drop-off in another country. You'll find contact details for Porto airport car rental operators here:https://www.aeroportoporto.pt/en/opo/access-parking/for-your-full-comfort/car-rental
Your best chance will be with multinationals Hertz, Avis/Budget, Europcar and Sixt, all of which also operate at Bordeaux airport (which does not, in itself, mean they will allow a one-way rental).
>any other consideration (with much flexibility) I should consider.
Portugal, Spain and France each have their own driving (and other) laws, speed limits etc. Make sure you are familiar with them beforehand. Driving in different European countries is not
the same as driving in different US states.2.
Make sure you have the equipment you are legally required to carry (e.g. a reflective jacket for each occupant + warning triangle for France).3.
Fuel pumps can be automated and card-only. Make sure you have a card which will be accepted in European countries. Some pumps won't accept any foreign cards so never allow your fuel to drop so low that you get stuck.4.
Don't underestimate how long it will take you to get from A to B. Map distances are deceptive. Rural roads can be narrow and winding and major roads can be very congested, especially in and around cities. Allow yourself plenty of time for each segment of your trip.