What are the MUST's in Barcelona for 2 full days?

What are your first thoughts? which is your initial list?

A "must see" list is different for every person. We can name popular sights but you have to choose which ones interest you. You'll find these sights in any guidebook. Some of the more popular ones are:

Gaudi buildings like Sagrada Familia, Casa Mila, Casa Batllo.

Walking Las Ramblas.

Shopping along Passieg de Gracia.

The cathedral.

The Picasso Museum. The Miro Museum.

Bari Gotic and Moderniste Architecture - one full day devoted to each.

Sagrada Familia...get your ticket in advance and avoid the line which can be very long. Go inside and just sit in one of the chairs where the church pews would normally be found and enjoy the details of the structure. Amazing for sure.

Sagrada Familia...get your ticket in advance and avoid the line which can be very long. Go inside and just sit in one of the chairs where the church pews would normally be found and enjoy the details of the structure. Amazing for sure.

Nou Camp soccer stadium tour. After Sagrada Familia it's the second most popular attraction in the city. It's a sprawling museum about the city's world-class team and it's connection to the region. Even if you're not a soccer fan you'll find it interesting.
Also enjoyed the Museu Nacional d'Art de Catalunya. Excellent collection located on a hill with commanding views of the city.
Barcelona is a great town but be sure to wear your money belt or use the hotel room safe as it's the pickpocket capital of the world:(

I would recommend doing the hop on hop off bus tour. Visit Park Guell. Sagrada Familia, Picasso Museum
walk the Ramblas and visit the Cathedral. That's a great list to start with and build from there. Tapas experience is also fun.

Sagrada Familia is #1, for sure! Hop on, hop off bus is great for an overview of the city. Walking Las Ramblas down to the Christopher Columbus statue, visiting La Boqueria along the way, and tapas in the Gothic Quarter are must-do's. Depending on your interests, the Picasso Museum or Antonio Gaudi's Casa Mila would be next. Have an amazing trip!

I should have expanded a little.

A day in Bari Gotic would include Las Ramblas and the beaches (essentially pick the sights in the area that most interest you). The walking tour from the TI would be a good start for the day.

Moderniste architecture would include the Passeig de Gracia, Park Guell, and Sagrada Familia.

Palau de la Musica Catalana