Half a day is plenty of time to get a feel for Ensenada [1]. If you have a car, you might spend a few hours or an entire day visiting wineries along the Ruta del Vino [2].
Anglers will be content to spend a week or more plying the waters of Bahía San Quintín [3].
For sierra-bound visitors, the best hiking seasons are mid-April–mid-June (good for wildflowers) and late September–early November (when quaking aspens put on a show).
Links:
[1] http://www.moon.com/destinations/baja-cabo/ensenada-el-rosario/ensenada
[2] http://www.moon.com/destinations/baja-cabo/ensenada-el-rosario/valle-de-guadalupe/ruta-del-vino
[3] http://www.moon.com/destinations/baja-cabo/ensenada-el-rosario/san-quintin