Montjuic is well known for its many little and big treasures it holds. You could dedicate a whole weekend to exploring the attractions on offer, from amusement park to ancient castle, from botanical wonders to spikes and thorns, from traditional Spanish village architecture to modern German lines and squares. When people mention the park on Montjuic they normally refer to the botanical gardens found near the Palace which acts as art museum today and seldom does the cactus park get a mention in all the hype around mountain.
Yet it is unique in its kind boasting 800 cactus species surviving in a microclimate on the east side of the mountain. You can leisurely wander through the park in an hour admiring the assemblies of different cactus species. Clusters of round cactus that resemble a colony of Alien eggs just about to hatch with flowers growing on top, or the cactus forest densely populated with over ten feet high cactus creating a surreal forest atmosphere. Beware of the cobwebs while passing through it! On the southern end of the park is a lovely wine leaf overgrown pergola where you can sit on some benches with a sandwich or two and look out to the sea.

The Park has several entry points, one at the bottom close to Av. Parallel and the Metro, and the other at the top at Miramar, the viewpoint of the sea with a little park and the cable car station. The entrance/exit at the bottom of the hill is in dire need of attention and thus has attracted the wrong kind of attention from the more unfortunate in Barcelona, with burn marks from nightly bin fires and numerous Xibeca bottles amongst other human and non human debris, evidently from the now habitual inhabitants of the lower side of Montjuic Park adjacent to Ronda Litoral.
The top entrance at Miramar has a wonderful view of port and the sea where you can overlook Port Vell, Colon, Maremagnum, and admire the industrial port with all its cargo and cranes.
To get to the park you have several options each with its own little perks. You could for example take the cable car from Barceloneta all the way to Miramar and back for 7.50 € and enjoy unique views of Barcelona, often described as flying over Barcelona with a plane.

You could also take the ancient and restored funicular railway from Paral.lel to the top of Montjuic and walk down to the park. Costs no more than a metro ticket and more fun than hiking up the hill.
Then you could take the special Montjuic Bus, Nº 61 that starts at Poble Sec, goes past Pl. España and Poble Español, up to the stadium, and back down towards the park and castle
Or finally you can embrace the mountain by walking up Nou de La Rambla until the top, take a left turn and head towards Miramar and the cactus Park on foot.

While visiting Montjuic remember to pop by at following other attractions: