As already suggested, if you want to spend two days I would visit both the Yarra Valley and the Mornington Peninsula. They're both about an hour+ drive from Melbourne (and in 'opposite' directions, so you won't feel like you're repeating the trip on day two). We live here and we've only ever done day trips.

Given that there really isn't much to "see" at a winery, it usually comes down to the buildings, the cellar door experience <some enjoyable and engaging, some impersonal>, the wines etc., the food and whatever else may be on offer.

In the Yarra Valley, we usually start with coffee and a cheese tasting plate at the Yarra Valley Dairy Company, then round the corner to Domaine Chandon, which has a good self-guided tour, which is worth the 15 minutes or so it'll take you to wander around and then a tasting flight ($) of their sparkling wines in the tasting room/restaurant. Then on to TarraWarra to visit the Art Museum, followed by lunch there, or perhaps at De Bortoli or Tokar Estate or Yering Station. You could also spend some time in the afternoon at the Healesville Sanctuary, if that is of interest.

A number of the wineries will give you a pretty decent lunch - perhaps worth checking out the menus online ahead of time and, depending on when you're going, probably need to make a reservation. One restaurant that has become very 'fashionable' is Ten Minutes By Tractor on the Mornington Peninsula - definitely make a reservation well in advance if you want to go there.

The tasting rooms vary, in part depending on how busy they are. One we always enjoy is Tuck's Ridge on the Mornington Peninsula - we always get the same guy in the tasting room and we always seem to end up going home with a case of their reds .... not quite sure how it happens.... Try their Buckle Pinot Noir...

http://www.yvd.com.au
http://www.chandon.com
http://www.tarrawarra.com.au
http://twma.com.au
http://www.tokarestate.com.au
http://debortoliyarra.com.au
http://www.yering.com
http://www.zoo.org.au/healesville

http://tucksridge.com.au
http://www.tenminutesbytractor.com.au