My city-visit planning normally consists of just noting what I want to see and each sight's opening hours, but Barcelona has a bunch of sights for which you may be stuck in very, very long lines if you arrive without a ticket. Or you might not be able to get in that day at all. Basically, if it's the Picasso Museum or a major Gaudi sight (La Sagrada Familia, Parc Guell, Casa Batllo, Casa Mila/La Pedrera), you should try to buy tickets at least one or two days ahead of time. (The Picasso Museum might need to be booked a bit earlier; it tends to be unbelievably mobbed and should be skipped if you aren't interested in Picasso's work.) I bought all my tickets after arrival, but I was in the city for 10 days and had a lot of flexibility. For the sake of convenience, I got the purchased-in-advance tickets from the tourist office under Placa Catalunya, which charges an extra 1 euro per ticket. But you can buy online if you prefer.
La Sagrada Familia requires a separate fee if you want to ascend one of the towers, and that comes with a separate entry time. The T.O. only handles the basic entry fee, so if you want to see one of the towers up close and personal, you should go elsewhere for La Sagrada Familia tickets. Booking the tower about 1 hour after the church should be about right.
The Palau de la Musica Catalana can be visited only on a tour or to attend a performance. English-language tours are pretty frequent.
I know nothing about the Camp Nou Experience (soccer-related) but wouldn't be surprised to hear that it sells out early.
Other than the above (and you probably will only be interested in a few of them), Barcelona is great for wandering around. You can make a spur-of-the-moment decision to go to Montserrat, Girona, or one of the beach towns if you decide you'd like a change of scenery. There's tons to keep you occupied in Barcelona, however. I loved walking around the city, observing the exteriors of a bunch of modernista buildings that are not open to the public.
I recommend that you stop in at the tourist office under Placa Catalunya at your earliest convenience. It has an unusually large number of English-language brochures on display for self-selection. You may run across something that aligns with a special interest of yours.