I spent 3 of the greatest weeks of my life in Spain summer of 95' (as well as a trip up to Assen Holland for the Dutch GP)
I would highly reccomend they take a trip to Segovia Spain
http://www.segoviamint.org/english/segovia.htmIt has a world famous castle(Alcazar castle,arguably THE castle Disney modeled his after), an incredible 16th century cathedral and a 2000yr old Roman aquaduct
Segovia is certainly a Spanish jewel. Your parents will love it. Well worth the bus ride or rental car fee.
Too bad they aren't going around the beginning of July they could visit Pamplona and run with the bulls
Of course when my friends and I were there we spent all night getting slappy drunk off Calamacho's (sp) (Spain's signature drink..50/50 red wine and coca-cola)
We ended up just watching the bulls enter the stadium at the end of the run. (there are several runs a day for a week, so the "running" isn't just one run)
They will certainly want to try some authentic Gazpacho (not these fake concoctions martha stewart and the like publish)
While in Madrid there is nothing like a good cheap calamari sandwich and a beer while people watching.
And where else can you walk up to a bar out on a median strip on busy 8l ane street?
You might also let them know that picking up some X from a stranger in downtown Madrid is really easy and makes the marathon long night of technodisco clubs more interesting
Once again where else can you leave a club at 7am walk across the street and enter another that is just opening up.
Oh and if some SUPER HOT woman wants to dance with your dad.. remind him that her boyfriend is probably the guy behind him trying to rub his *ss on him.