You should buy tickets for Parc Guell, Casa Mila/La Pedrera, Casa Batllo and La Sagrada Familia in advance. Ticket lines at those sights are extremely long. I don't know whether any of them are currently selling out days in advance.
I know nothing about Camp Nou itself, but there used to be some special option, called "Camp Nou Experience", that was said to sell out way ahead of time.
The Dali Theatre-Museum in Figueres hasn't historically sold out way ahead of time, but it does get bus tours. I'd check ticket availability several days ahead of time to be sure it looks OK, then I'd buy the ticket no later than the day before the planned visit. On a day trip it would be awkward if you showed up without a ticket and were sold one for a time 3 hours later. Figueres, frankly, doesn't have a lot of sightseeing options. Girona (between Barcelona and Figueres) is a totally different story, but you'd need to buy two separate train tickets if you wanted to stop off in Girona for a while.
The Dali Theatre-Museum ticket also covers the jewelry collection nearby, which is fabulous. You can see the jewelry collection before your DT-M ticket entry time (or after seeing the DT-M).
Note that there are two train stations in Figueres. The fast trains from Barcelona and Girona go to Figueres-Vilafant, which is close to a mile from the Dali Theatre-Museum. The slower trains go to the Figueres station, which is closer to the DT-M. There's a city bus connecting the two stations and passing within a few blocks of the DT-M. It seems to be timed to train arrivals at Figueres-Vilafant, but if the train is late, the bus doesn't seem to wait. If that happens to you and you don't want to walk (about a mile) to the DT-M, don't dawdle inside the station; go right outside to see whether there's a taxi there.