After going up to the rooftop of El Circulo de Bellas Artes I'd have to vote for that. And at 2 Euros and the views they offer, I'd say it's a must-do.

I also like to recommend the Teleférico between Argüelles/Parque del Oeste and the Casa de Campo, crossing over the Río Manzanares for some nice views.

Along the same lines, when it finally re-opens in September after years of renovations, going up to the observation deck of El Faro de Moncloa will offer good views too.

As for things to buy, well, even when I came to Madrid as a visitor I never bought anything to take back. I guess a Spanish fan (one of the good, Spanish-made ones, not the ones from China) would be a good gift, though.

Saludos, MadridMan