Hina Nagashi is a custom that originated in the Heian era, and is intimately linked with the famous Tale of Genji. In the celebrated novel Genji takes part in a ceremony where the traditional hina doll, usually given to female children as a way of instructing them in housekeeping and wifely ways, is cast into a fast-flowing river as a proxy for its owner. The idea was that the cleansing properties of the waters would purify the owner of sins as well, allowing them to move forward with their lives.
Currently the dolls for hina nagashi are made of paper, and set afloat in little boats that are later recovered (to prevent damage to the ecosystem), but the tradition survives. For those who would like to give it a try, there will be a hina nagashi conducted at Kairakuen park on both February 26th (11-12) and March 3rd (10:30-12, the actual festival day), at the little retention pond to the southwest of the main park (north of the train tracks).
For those who prefer to remain indoors, there is a display of vintage hina dolls at the Mito City Museum, or you could visit one of the many Hina Festivals across the prefecture, such as the famous festival in Makabe.