I stayed at a private flat that I had somehow found on the internet
(some 7 years ago). It was close to a bus stop that was a 5-10 ride to
school. My friends did not have the best experiences at the places the school suggested that year since they were meant for traveling students although they were a little closer to school.
I got there early before my program and stayed one night in a pensiones, which was reasonably cheap and by the downtown park area near the water. It may be a budget option to a hotel since I would imagine that you would probably want to spend most of your time enjoying the locale. It is definitely better than a hostel. You may want to do a google search to try to reserve a place beforehand, but when I was there there were always vacancies at pensiones.
I basically found everything through my Spain travel guide book that was similar to Lonely Planet. It listed some of the popular sights, eats, accommodations, and happenings. It even had some common Spanish phrases which came in handy since I did not speak one lick of it.
Santander is not very big, so you could do a lot of walking everywhere with help of the bus or cab if needed. I would suggest that you rent a bike for your short stay if possible.
It seemed like a safe place when I was there. Guys and girls would stumble back to their places alone without incident near the schools. If I remember correctly, there may have been a red light district, but I do not recall going there.
Since I was a super poor graduate student back then (even more poor than a college student), I did not eat out much. I did eat a lot of bocadillos and tapas while I was there with vino tinto since that always seemed cheaper than soda or even bottled water. I can't remember where I we ate, but the fresh fish and locally grown beef were good when we actually did eat out.
My trek to Santander was by way of bus from Madrid. There is a small Santander airport as well as a Bilbao airport. There are a bunch of small discount airlines like Ryan Air, Easy Jet, etc that made travel easy and cheap.
For places to see and things to do, check out an updated Spain travel book at your local book store. I only used it initially when I got there for the main places to go and sights to see, but after I usually just explored.
Actually, I just did a wiki search and all the main things to see are listed:
http://en.wikipedia.org/wiki/Santander,_CantabriaEnjoy! I hope to visit Spain again soon!
Jude