Behind the Pena National Palace is a botanical park which is really a forested area of 200 hectares, created at the same time as the palace. Trees from many lands were brought to be planted there - from North America, to Asia, to Australia. Many types of ferns were also brought and grown in the Queen's Fern Garden. The forest is very fascinating, with hundreds, perhaps even thousands of different species, whose growth the special or microclimate of Sintra allows, and designed in labyrinthian paths on an uneven terrain, all of which adds to a sense of mystery.
There is one particular tree in the park that i love, the North American sequoia, which is just awesome to see. With its sheer size and magnitude, it's impossible not to feel respect for this living being who has kept standing and growing through centuries of events.
Most people only visit the Palace, but without visiting the park, I think one can only experience half of the enchantment of the place.
Sintra has so many interesting points to visit, that most tourists visit the palace and skip the park. If you have time, don't do it.
The park has a great botanical variety, and some views are great.
Remember - the ticket to the palace includes the park, and you may also visit only the park. If you are visiting also the other monuments check for a combined ticket and save.
Take a hike around the park surrounding the Palace of Pena and climb up to St Catherine's height for a good view of the palace from afar. A little creepy though as the climb's through a small narrow windy path and there's absolute silence surrounding you at the top - hardly any hikers went our way during our climb and rest there.