You can do Sintra without a guide if you’re comfortable planning logistics and managing timing, but that’s where most people underestimate it. Sintra isn’t hard to navigate, it’s just very time and energy consuming because of terrain, distances, and queues.
If your goal is only to walk the gardens of Pena and visit one or two sites at a relaxed pace, it’s doable on your own. But stacking Pena, the Moorish Castle, and Regaleira in one day involves a lot of uphill walking and stairs, even if you skip interiors.
Where a guide really added value for us was efficiency. We had a private guide, Daniel Ponce, and the experience was excellent. Knowing which site to do first, what to skip, how long each place realistically takes, and how to avoid dead time between stops made the day flow smoothly instead of feeling rushed.
Sintra easily fills a full day. It’s just much more enjoyable when you plan around terrain and crowd patterns rather than trying to see everything.