Hi,
You can also see the city by the turistic bus: http://www.bcn.travel/tours/barcelona-hop-on-hop-off-bus-turistic?gclid=CLOWt4uCzbkCFZShtAodT2cAyA
A good place for the proposal is the Tibidabo at night, from there you have incredible views from Barcelona. You can go there with the excuse of cache:GC5A26
Enjoy your trip!!!!