Ely is quite a small city, so a lot of it can be explored comfortably in a short stay. It's a pretty place with a number of old buildings. The most famous is the cathedral, which dates back to 1109 but in fact there were religious buildings on the site much earlier. Ely also contains the house where Oliver Cromwell lived for 10 years, and the house has been restored to give a real picture of how he and his family lived in the 17th century. There are lots of other historic buildings too, so for history fans Ely is a great choice.
You can also go walking or cycling in the Cambridgeshire countryside, or visit famous nearby places such as Cambridge. Like all cities, of course Ely also has its share of night life, places to eat out etc, but history and countryside are what make it special. For more ideas on what to do in Ely you can look at the tourist information site.