We are going on a cruise and will be in Barcelona for 12 waking hours. This is divided into 2 days. What should we do? Montserrat looks amazing. Should we just do the highlights the first day and Montserrat the second day. Cruises are just a taste.
I would spend the whole time in Barcelona. Buy reservations/tickets ahead online for the famous sites and Sagrada La Familia.
Montserrat IS amazing. Barcelona is pretty amazing, too. I would see your sites the first day and decide at the end of the day if you need more time in the city or if you want to do Montserrat. It is just a question of priorities and for Barcelona, that'd be a tough one. You don't need to buy tickets in advance for Montserrat. You probably need to purchase tickets in advance if you want to visit Park Guell, Casa Mila and others, La Sagrada Familia, museums, etc.
With Montserrat too much time will be wasted on travel--unless you rent a helicopter.
Consider Montjuïc. https://barcelonando.com/montjuic-olympic-ring
You must buy Sagrada Familia tickets in advance and I recommend that you purchase the 0900 time.