You can not come to Barcelona and miss La sagrada familia , Park guell, Casa Mila and Casa Batllo Those are the most important monument.
The only handicap is that the line to visit those attractions is endless. You can spend 2-3 hours just waiting in the line to buy the ticket. I highly recommend buying tickets in advanced.
Some times ticket can be a bit more expensive maybe 1-2 euro more… but I promise you its you will be soooo happy that you are skipping the line while others are just waiting and waiting