A very pleasant option is to walk uphill from Herrsching to the village of Andechs. An added advantage: in Andechs, you have a fine monastery that brews magnificent beer. There are also bus options to get back down the hill should the weather turn bad (or you consume a beer too many... ;)
There are two paths to get to Andechs: one via the "Horndlweg," and one via the "Kiental." The former is a bit longer (roughly 1 hour and 15 minutes), but has better views in clear weather-- sometimes with views of the Alps! UPDATE: When we walked the path in May 2008, the Alps were indeed out! Enjoy the new photos! 8). The Kiental path takes about an hour. It's a more woody path following the stream and with a long walk along a pleasant pasture. Sometimes, the cows will come out to check on your progress.
One activity that looked like lots of fun was to take a boat ride on the Ammersee. The boat departs the Herrsching dock several times a day. You can purchase various round-trips, or use the boat as transportation to get to the other side of the lake. In fact, we planned a scenic detour en route to Lindau by taking the boat from Herrsching to Diessen, then a regional train to Geltendorf, then on to Lindau. Unfortunately we awoke to heavy rain and dense fog, so we took the more conventional S-Bahn to Pasing and connected to our Lindau train.
As of 2010, fare is EUR 16 for a "grand tour," EUR 12.80 for a "northern round trip," and EUR 10 for a "southern round trip." There are discounts for large groups, as well as a 10% discount with an MVV Tageskarte (Gesamtnetz or Aussenraum), CityTourCard (though note this more expensive ticket gives you no greater discount than a standard MVVV Tageskarte), Bayern Ticket, or Werdenfels Ticket. You might also get a discount with a railpass if you smile and ask nicely. Then again, you might not.
Herrsching has a very pleasant walking path along the Ammersee. Even if the weather is cloudy (or even a bit rainy), it's worthwhile to take the time and wander along the shore. The views are enjoyable, even if clouds are overhead.
WALK THROUG THE FOREST TO THE MONASTERY OF ANDECHS , THE WAY IS SIGNED AS THE WAY TROUG THE KIENTAL !! AT THE MONASTERY U LL GET THE FAMOUS BEER , BUT D ´ONT DRINK TO MUCH FROM THE " STARKBIER "