I have visited Anuradhapura almost six times now and each time its not that hard to find a new place that I haven't visited before. If you are visiting Anuradhapura then you should plan it before hand. If you are a tourists a tour guide is highly recommended.

If you are a tourist visiting all the pagodas might make you boring because you don't know the historical facts attached to them. If you are there for a one day visit I can suggest the following.

The Ancient bo tree

Ruwanweli Maha seya ( pagoda, considered the best in SL )

Moonstone in Queens Palace

Kuttan Pokuna ( beautifully carved pond )

Samadhi statue ( beautiful statue of Lord Buddha )

Isurumuniya Temple ( beautiful stone carvings )

When it comes to tourists my experience is they are mostly pleased at visiting Isurumuniya, because of the beautiful stone carvings.