No one can visit Barcelona without walking / exploring its very old heart for some hours and... on foot! the Plaça del Rei, the Cathedral, the Roman Walls, the Borne passeig... However for a sightseeing tour to and through other important destinations in the city the BIKE is easy, quick and fun, is also a great way of seeing all of the great architecture that you normally would miss using the metro. Separate cycling paths form an essential part of the Barcelona traffic network. Many of them serve as a separate lane for cyclists along main traffic roads, these roads themselves are therefore invariably forbidden for bicycle traffic. On the other hand, there are numerous touristic cycle paths providing additional opportunities that cars don't get! Very pleasant are the touristic cycle paths leading through gothic quarter and seafront (see below).

The risk of your bicycle getting stolen merits an attitude of caution, as bike theft makes up a large portion of petty crime in Barcelona. However, there's no real need to get too paranoid in general. Always lock your bike up and leave it in a place where there are many people. Lock it to something that's permanently fixed to the ground and that's rigid and difficult to break. Use two locks: one to lock the front wheel and the frame to something fixed and rigid, and the other one to lock the saddle and back wheel to the frame. Consult the map of the network of cycle paths and find your route!. Our route recommended is: 'Seafront Barcelona'

Our outing is to the sea, leaving the bike lanes once in a while to venture into areas where we can get a breath of real salt air: Seafront Barcelona, an excellent place for a day out cycling. We start the day on the terrace of the inviting cafe-restaurant of the Centre de Cultura Contemporània de Barcelona (CCCB, Montalegre, 5), entering through the CCCB or Plaça dels Àngels. On leaving the CCCB, we might stop to watch the acrobatics of the skateboarders in Plaça dels Àngels, in front of the Museum of Contemporary Art of Barcelona (MACBA). Leaving our bikes locked, we can walk down to Carrer del Carme and turn left to the new Rambla del Raval, the heart of the Raval neighbourhood and undoubtedly one of the most culturally diverse spots in Barcelona. With so much immigration, the Rambla del Raval, is decidedly exotic, and may come as quite a surprise to those who find themselves there by accident on a Sunday stroll.

Getting back to the bike lane, we ride down Avinguda de les Drassanes to Passeig de Colom. Here we find the celebrated golondrinas, the tourist boats. Turn right to the quay, Moll de Barcelona, where a broad avenue takes us to the middle tower of the cable car to Montjuic, and a more modern construction, the World Trade Centre, which has an interesting fountain in the courtyard. If it is a holiday, the building will be practically deserted. We go back to Passeig de Colom and pedal on to the Moll de la Barceloneta, then venture into the lanes of Barceloneta, the fishermen's neighbourhood in Barcelona, to discover the charm preserved in its narrow streets, full of children playing and clothes hanging out to dry. No, this isn't Naples; it's 21st century Barcelona. It is good to stop for a traditional vermut with his 'tapas' at any of the bars by the Market, for example La Cova Fumada (c. Baluard 52), whose house speciality is bombas with all i oli , a hot garlic and olive oil sauce. Before lunch, we have a few minutes to relax on the sand at Platja de Barceloneta, where most of Barcelona's summer tourists invariably end up. This is undoubtedly the spot in the city where there are more nationalities per square metre. For those brave enough, there is a stretch of sand for nudists at Platja de Sant Sebastià.

Getting back on our bicycles we can cycle all along Passeig Marítim. There is the choice of pedalling along the beach or along the bike lane, flanked by palm trees. In a few effortless minutes we will pass in front of the Hospital del Mar and soon after under the two high rise buildings of the Vila Olímpica (Hotel Arts and Torre Mapfre) and the immense fish sculpture designed by the same architect who designed the Guggenheim Museum in Bilbao, Frank Gehry. Here there are dozens of places to eat. Then, to help digest our food, there is an easy ride to Bogatell Beach. Here, there are fewer people on the beach than in Barceloneta, so it's a good place for a dip and, of course, a siesta. The two chiringuitos (beach bars) opened here a few seasons back are open till the early morning hours and have become a preferred outdoor hangout for nighthawks. (Via bcn.es) 

There are different companies in the city to rent a bicycle and tours:
Bike Rental Barcelona - They deliver the bike directly in your apartment (from 9,99 €).
Un cotxe menys - Rent bikes by hours, by days or by weeks. 
Bike Tours in Barcelona - Daily english tours in 3 hours.