I have made the journey several times and the most convenient Ibis Hotel is the Ibis Madrid -Alcorcon-Mostoles-go to their web site
Your drive from Santander will take you on to the M50 and this Ibis is only a few minutes from the M50. close by there is MacDonalds and Burger king for a cheap meal otherwise you can walk in to the centre of Mostoles in a few minutes-I think this is more convenient than the Ibis near the airport
There are two Ibis Hotels in Mostoles and it is the one above which is the most convenient
It is located behind a BP petrol station and is a bit tricky to find so either use Sat Nav or down load a route from Santander to the hotel from Google. also going South Google a route as it can also be a bit tricky to make sure you rejoin the M50 the next day South!
Breakfast is very good and we always take a roll for lunch !
I found out about this Hotel from a post on EOS!
Enjoy your trip