Spain
Spain's a great place for loafing around on the trains and buses. Here are some ideas.
Getting there (from the UK) and around
Train
For details on getting to and around Spain by train, as always, my first stop is The Man in Seat61: https://www.seat61.com/spain.htm
Eurostar to Paris, then:
TGV to Barcelona (or Figueres or Girona)
TGV to Nimes (or Avignon) then TGV or TER to Cerbere/Portbou
Night train to Cerbere
Night train to Latour-de-Carol and train to Barcelona
TGV or night train to Hendaye
Check out Rome2Rio for the more obscure options.
Coach
You can go by coach to Spain changing at Paris Bercy, so if trains aren't working for you, this is an OK plan B. Coaches take around 12 hours from Paris.
(in fact it's how I first went to Spain as a kid).
Tourist Info
Guide books/sites
Rough Guide: https://www.roughguides.com/spain
Lonely Planet : https://www.lonelyplanet.com/spain
Trip advisor: https://www.tripadvisor.co.uk/Tourism-g187427-Spain-Vacations.html
Accommodation
If getting your place through booking.com, there's a Travel Sustainable option.