You cannot go wrong visiting Madrid and Barcelona. Other popular cities in Spain are Sevilla and Granada.

But with 6-7 days, I would recommend visiting only one or two. This will cut down your travel time to your next destination.

You can visit Madrid or Barcelona and there is plenty to see and do for a week in each city. If you feel you want to visit another city, Barcelona and Madrid are completely differnet cities, you can find cheap flights between these two cities.

You can find inexpensive places to stay in Madrid/Barcelona, but depending when your are coming that could be a problem to find places that have room available.

Bill