I loved the 24 hour ferry (santander to Portsmouth) when I came back from my Pyrenees tour in July.
Mind you, it didn't hurt that we were in a Commodore class cabin so we got to sit on our private balcony and enjoy the bay of biscay being flat as a pancake.
A lot of the time travel sickness is made worse by tension - so if you worry about getting sick, you will get sick. As well as medication and trying to keep an eye on the horizon it really help to have a way to distract yourself, anything that takes your mind off it if you start to feel a bit rough. Booze is not good for this, but music, audio books or Paul McKenna are all good.
Northern Spain is gorgeous, even the motorways are fantastic - all twisty roads following the mountains and coast with lots of tunnels for being immature and noisy in (suited me perfectly).
As for general touring advice, take less kit than you think you should. Make three piles - essential, desirable and luxury. The essential kit is everything you must take - passport, phone, docs, cards. Everything else is optional, its mostly only worth taking kit you "expect" to use rather than things you "might" use. If something is cheap, readily available and you might not use it anyway - then why not just buy it down there if you need it rather than drag it around with you. I always take a load of old socks, boxers etc so I can bin them when they're dirty near the end of a tour - leaves room for gifts, souvenirs and cheap cigs!
