Don't be suprised if you come away from Montserrat as I did - feeling like the mountain itself was much more impressive than the monastery! Should I ever return, the entire effort will go towards further exploring the hiking trails.
Once you deboard the funicular, it's a short walk up to a central location where a nature center is located also containing geological information about the area. You'll also note signs marking three trails, but without rhyme or reason to which you should tackle first. I suggest first taking the Geroni Trail which heads off to your right/south.
The trail is rather easy with level grounds to traverse. You'll walk thru some wooded area before the paths open up allowing open-air vistas looking across and back down to the monastery area. By now you'll be standing at the base of the unique peaks which you should try to identify from below before making the trip up. I stopped and gawked for quite some time before heading on...though I couldn't determine exactly how much farther. The trail is rather deceiving the way it continually curves and disappears. You'll be tempted to keep going just to see what's there. I went well past where the monastery was no longer visible. Should you desire, various trails lead all around the southern parts of the mountains.
Once you've returned to the nature center, there's also a lesser trail which is forward and a little to the right. I didn't walk this one, but along the way is a cave area where a monk lived for quite some time.
From this central area, the main trail (don't recall its name) is to the left of the Nature Center. I would rate this trail at more of a moderately difficult level with a few hills to climb, but it's all mostly downhill with slippery pathes from loose rocks. Views around the mountain and across the scenary continued to inspire me for "stop and stare" breaks...even on this overcaste day. I couldn't see the Pyrenees or Mallorca, but Tibidabo point in Barcelona was visible.
Along this trail are several small chapels, memorials, crosses and significant religious relics you can stop to see. Some are directly on the path; others quite the distance off. If you continue in the downhill direction, the path will eventually pass the campgrounds and deliver you back to the monastery near the funicular.
If you've time to venture onto only one of these trails, I highly recommend Geroni based on the easy level and spectacular views. You'd be wise to ride the funicular back down. As for the crowds, less than 20 people passed me on the Geroni trail...thankfully, as it was too narrow for any tourist buses.