Take the metro from La Sagrada Familia up to Lesseps - Parc Guell is signposted from there. Intended for wealthy families to buy triangular plots of land and have their houses built there (perhaps by Gaudi, perhaps not) the venture never took off and another one of Gaudi's ideas fell by the wayside.
The park's attractions include its serpentine ceramic bench, water fountains and the Hansel-and-Gretel-style houses that Gaudi did manage to finish.
Entry to the park is free. See more about Free Things to Do in Barcelona
Gaudi Trail in Barcelona
Placa Reial (Plaza Real)
La Pedrera (Casa Milo)
La Sagrada Familia
La Sagrada Familia - Up Close
This walking tour of Barcelona takes you to the major modernist architecture in Barcelona, from the works of Gaudi to the likes of Domenach i Mutaner and Puig i Cadafalch.Buy Direct
If it's specifically Gaudi that you're interested in (or you don't want to walk so far), this Tour of Gaudi's Barcelona
is ideal. You'll take in Placa Espanya, the National Palace, Guell Park and the Sagrada Familia over an eight-hour period. This is a good introductory tour of Barcelona with some of Gaudi's highlights to be seen, but I wouldn't say it that closely linked to Gaudi. The tour above is better.