There are many options for day trips from Lisbon, but Sintra is not to be missed. In the high season I can imagine its even more crawling with tourists but the beauty of the region is worth it. If you use public transportation its easy to take the commuter train to Sintra, it takes about an hour. *Tip: buy your tickets in advance in order to avoid the long lines to buy tickets in the morning. The train (in March at least) only left once an hour. I would arrive early as well in order to get a seat, especially in the summer I think it can get really crowded and you’re in for a long day of walking!