It was inevitable, I suppose, but the long-embattled Ramblas has finally succumbed to the hen and stag invasion. If it’s a weekend anywhere near summer and you’re not wearing an L-plate or a pair of fake breasts, you may as well forget it. Because this is a street caught perpetually in the mood of chucking-out time in binge-drink Britain. Mainly because it’s full of binge-drink Britons, all thrilled that Barcelona doesn’t really have a chucking-out time at all.

That is the bad news. But don’t panic — the hunting packs confine themselves to that part for their gutsy olé olés, so the rest of the place is all yours. And it gives up its secrets with near tartish ease. Wander down any Barri Gotic backstreet and you can’t help but happen upon a saucy little tapas bar. Or a cheeky little art gallery. Or a fruity little wine bar. Or something that madcap genius, Gaudí, got his mitts on. It’s a hot, sexy city with maze-like alleyways in the old bit, wide-as-Paris boulevards in the less old bit, knickers hanging across streets in the rough old fishing neighbourhood and, of course, a beach. There are pretty girls on bicycles and pretty boys on mopeds, fishermen’s wives doing fine things with fish and old bartenders pouring you another glass of crisp cava, even though you said “No” because one’s already a bit lush before 11.

It’s all one big seduction. You might look like a big, pale north European, but after a day or two, you won’t feel like one. The salty air, the tapas, the sun-kissing won’t allow it. But there is one big trick to Barcelona: don’t follow a plan. You mustn’t rush, you mustn’t sightsee and, beyond nabbing a flight and a hotel, you should on no account have an itinerary. Just set off with the vaguest idea of direction, eat when you’re hungry and look at Picassos if and when you feel like it.

So, what’s the point of this guide? Purely to aid your serendipitous progress. Nothing obligatory — just a few things you should, perhaps, if you feel so inclined, on the off chance,make sure you stumble upon.

Three tapas: they are almost all good, but here are my three favourites. Start at Sagardi (Carrer Argenteria 62; 00 34-93 319 9993), in the Barri Gotic. The system is simple. Order a drink, ask for a plate and stand at the bar, munching through the food mountains. How do they know how many you’ve had? They count your toothpicks.

Next, walk north into Eix- ample and stop at Ciudad Condal (Rambla de Catalunya 18; 93 318 1997), a more traditional establishment with flasher morsels. And finish up the road at Cerveceria Catalana (Carrer Mallorca 236; 93 216 0368). Sit at the seafood counter, point and eat: the squid, prawns and unrubbery calamari are all amazing, but the whitebait in tempura (£2.50) is worth writing home about.

Three Gaudí: the city’s artist-saint was run down by a tram in 1926. The tram driver thought he was a tramp, but once the authorities identified his body, he was given a funeral fit for a king. The city is peppered with Gaudí’s fairy-tale flourishes, but there are three creations you must not miss (bang goes serendipity for this section).

First, the Sagrada Familia (Carrer Mallorca 401; 93 207 3031, www.sagradafamilia.org). It is still a building site 120 years after work began, but that doesn’t diminish the power of Gaudí’s trademark spires. Get there at 9.50pm, not a moment later, when the frankly terrifying facade lurks in darkness. At 10pm, hallelujah! The lights come on, and even the most committed atheists go: “Wow.”

Second, La Pedrera (Passeig de Gracia 92; 93 484 5900; daily, 10am-4pm; £5) is an art-nouveau apartment block that just doesn’t make sense: there are no straight lines. It’s one big blessed relief to see that even something as conformist as an apartment block doesn’t have to conform. On weekends, get there at 9.30am to avoid queues.

And third, Parc Guell (Carrer Olot, Casa-Museu Gaudí; 93 219 3811; daily, 10am-sunset; £3), a dreamlike playground of form and colour designed to blend with the natural landscape. A good place to stroll, adopt puzzled expressions at the barmy installations and pop into the eccentric house designed by Gaudí for Gaudí.

Arty pants: the Picasso Museum (Carrer Montcada 15-23; 93 319 6310; closed Monday; £3.50) is a big draw, but Picasso lived in Barcelona for only eight years, and there are better collections elsewhere. If the queue is bad, content yourself with a meander down the same street, browsing the excellent contemporary-art galleries. Then reward your diligence with a glass of champagne or home-made martini in El Xampanyet (No 22; closed Monday). It’s this sort of experience that you are here for.

Market day: La Boqueria (Monday-Saturday, 8am-3pm, 5pm-8pm) is just off the Ramblas, but that’s all right, because it is Barcelona’s best. Garlands of dried peppers and chillies adorn the rafters, while specialist stalls sell magical mushrooms, any type of nut and crystallised fruits. Stalls at the front can be overpriced, so head for the back, where the smart money is. After you have had a good browse, lunch awaits at El Convent (Carrer Jerusalem 3; 93 317 1052), where the food is culled directly from the market.

Get out of town: too much to squeeze into a first weekend in Barcelona, but second-timers or long-timers should consider a visit to the 16th-century basilica of Montserrat (Placa de la Creu; 93 877 7777; 7.30am- 7.30pm). Meaning “serrated mountain”, this is the spiritual heart of Catalonia. If you can stand the crowds, get there at 1pm to hear the Benedictine monastery’s boys’ choir sing Mass. Aled Jones times 20. It’s a one-hour train ride from Placa d’Espagna (line R5; hourly from 8.36am). From Aeri de Montserrat, take the cable car.