Barcelona is one of my favorite cities to visit in the world. There's so much to eat, so much to see, it's super walkable (and super cab or Uber-able as well)... and everyone is generally happy and friendly! On top of it all, you can get around fairly easily with English - which helps quite a bit when asking for directions (or more likely... ordering food). Of course... what this means is that it is near impossible to narrow down the list of recommendations!
So... I'm starting with just one day... one day of all my favorite things in Barcelona. It centers around the city center... and even though I've listed 7 stops on this itinerary, there's plenty to see and eat along the way.
Total walking miles: 6 miles / 9.6 km / 2 hours
(This estimate does not include any walking inside the museum)
(A) You know how everyone has told you to save dessert for last your entire life? That rule goes out the door in when you're in Barcelona. Start off your day with some incredible churros and hot chocolate at Xurreria Laietana. You need the energy for walking... plus it's only cultural to do so!
(B) I've been to Barcelona 3 times. The first two times, I got to see different parts of La Sagrada Familia because it was under construction for years. Finally, the third time, I got to see it in all its glory. Make sure you buy tickets in advance as the lines can get long... and even if you wait in line to buy tickets there... you're still buying it for a future designated time in the day. If helpful, there's a small soccer museum right across the street that you can spend 20-30 mins in.
(C) I wouldn't recommend doing La Sagrada Familia and Casa Mila in the same day. You must certainly can if you want to bask in Gaudi all day. But, you can catch a glimpse of Casa Mila if you take the right streets from La Sagrada Familia to lunch at Mercat de la Boqueria.
(D) Oh, the wonderful Mercat de la Boqueria! There's so much to eat! I can probably do a "One Day at Mercat de la Boqueria" if it was only open for the whole day. El Quim is probably the most famous place in there - and it more than deserves the #1 spot. But I've also eaten at 5 or 6 other restaurants there and I've never been disappointed. If you do get to make it to El Quim - make sure you get the baby squid with egg. I dream about that dish! Then wash it all down with a glass of Sangria or Estella on tap... or walk down a few stalls for one of the many freshly squeezed fruit juice stalls.
(E) After lunch, where you inevitably overstuffed yourself, it's time to do some walking. While you most certainly can walk from the Mercat to The Museu Nacional d'Art de Catalunya (30 min walk), the museum also provides a great way to walk off your calories indoor, while enjoying some world-class art. And if the morning walk before lunch was more than you needed and you're ready for a siesta, there's plenty of lawn space out front to sit down, rest, and enjoy the outdoor statues or some good old people watching!
(F) Even if you're not a foodie, you've heard of Tickets. It is an absolute incredible dining experience... and also impossible to get into. You have to make reservations months in advance (you might want to make the reservation ahead of booking your tickets... no pun intended). But if you can't get into Tickets... no worries, their sister restaurant - Bodega 1900 - is right across the street! Make sure you get the liquid olives (so good, so innovative, so memorable)!
(G) At this point, you should probably head home and rest up for tomorrow. But if you're feeling too alive to sleep (as one often does in Barcelona), then walk around the corner to Kyoudai 33 for a late-night cocktail. It may seem like a bad idea at first, but trust me, you'd be glad you decided to stay out after the first sip!