Bhaktapur, home of ethic community Newars, is the best option suggested by tour operators in Nepal for your day tour. Different spots of Bhaktapur like golden temple, Durbar Square etc. are very famous.
Your itinerary should not miss two options i:e:  Newari Home visit with traditional lunch and taking part in making pottery in Bhaktapur. I found this option to be the coolest and best travel experiences in Bhaktapur.