Given the amount of time I've been in Barcelona I've probably been to Sitges too many times, but frankly, I'm not sure there is such a thing for this picturesque beach town just south of the city. During the day it's filled with locals and tourists hunting for a piece of sand on one of its 17 beaches, walking along the boardwalk, or seeking out gelato, a coffee or a caña. In the evening it turns into a party town, having been dubbed "Little Ibiza." From what I've seen, there's always something going on in Sitges, so I'd suggest doing what I've done - check the weather, grab your suit and head over. If you're more of a planner, this website has great information about Sitges. In terms of events there's everything from an International Film Festival to a 5-day Pride festival, one of Europe's most intense Carnival celebrations, and even smaller events like this Wine Harvest Festival we happened across this past weekend:
For events by the month, check out this link.
Sitges boasts a climate always suitable for outdoor activities (300 days of sun!), although when I went last March it was awfully windy and pretty cold. To be fair though, it was sunny:
Since the weather was less than ideal, we had to huddle up and enjoy a delicious treat - Nutella waffle with whipped cream. Given we had to eat some before we could take the picture I'd say it was a close second to hanging out on the beach.
For a great day in the fall, winter, spring or summer, I recommend starting with some beach time.
After the beach, take a break in the afternoon for a snack of gelato, chocolate waffles, coffee, or one of the other delicious treats staring you in the face from every other shop entrance.
Then you can take a walk along the boardwalk. If you walk away from the Parish Church (Iglesia de San Bartolomé y Santa Tecla) up on the hill, you have 2.5KM (1.5 miles) of beautiful boardwalk that will take you to the Hotel Terramar, Hotel Punta Golf Course, and the huge Terramar park. Relax and enjoy the views. Take in the sunset from here or head back towards the train station and enjoy sunset from the church steps.
Getting to Sitges By car you'll have a beautiful drive and avoid a toll by taking the ocean route, but it will take you about an hour to get there and you'll pay to park. To drive direct you'll pay a €6 toll (and parking), so unless you're dying to take the ocean route or have a big group, I recommend public transportation. Train - The Renfe - R2S runs from multiple train stations within Barcelona. It will get you out to Sitges for a little over €4 in 30-40 minutes Free free to take the train back, but if you're planning to stay late, your best bet to get back to Barcelona is a bus. It's a longer ride back, but currently only €4.
Hope you enjoy! I know I will many more times.