Lausanne is ok, but I would get off one stop earlier and visit Morges, instead. Nicer lakefront stroll and plenty of cafes to stop for hot cocoa/light lunch. Attractive pedestrian street with nice farmers' market on Saturday mornings (good for snacking, too!) All easy to walk from the train station. Lausanne is bigger and has more shops, but not more than Geneva, and there is a larger "not so nice" part, especially around the train station. If you do go to Lausanne (which has a large farmers' market on Saturday mornings, too), try fondue in the cute little place in Ouchy just by the Beau Rivage hotel (can't remember the name), or splurge on a fantastic lunch at the Beau Rivage Rotunda (or more casual in the Cafe). Nicer than the earlier-mentioned Palace.

If you want to go a bit further than Lausanne, consider Lutry, Cully or Vevey - all smaller and cuter than Lausanne or Montreaux.