While the best man organises the stag do, the maid of honour will be in charge of the hen do. You may want to make it a complete surprise for the bride, or have her involved in the planning. Ultimately, the idea of a hen do is that it is fun, so make sure you get some ideas of things she would like to do as a starting point.
It is great fun being a maid of honour and standing side by side with your sister or best friend. The most important thing is that you enjoy it, and make sure that the gorgeous bride has the best day possible. She will be grateful for it for many years to come!