Drottningholm Palace is an accessible UNESCO site to visit, about an hour by public transport. If you make your way out, I would spend a good half day exploring the palace and grounds as they are quite extensive. When I visited in July of 2024, I booked a tour of the interior of the palace. Having explored many castles in London and Scotland, it wasn't the most impressive interior to a royal building that I've seen, but the history of Sweden is rich, and you get to experience that through the architecture, art, and stories that you hear. My one complaint was that the site to book tours on is difficult to navigate and did cause us some issues. However, we had a lovely tour guide who made the experience quite enjoyable. 

What makes this site so impressive is the large amount of land that this palace sits on. We spent a couple of hours walking around and exploring the gardens and neighboring woods. If you come to visit, I would dress for the weather as the best part is the grounds.