Question Budapest Must-See Sites and Attractions

I'm planning a trip to Budapest and would love to hear your thoughts on the must-see sites and attractions. I've heard about the Buda Castle, the Parliament Building, and the thermal baths, but I'm curious if there are any hidden gems or lesser-known spots that are worth visiting. Also, any tips on the best times to visit these places to avoid the crowds would be greatly appreciated. Looking forward to your recommendations!