Lauterbrunnen valley is roughly north-south, with steep sides. There is only sunlight for a few hours around mid day in winter.
Grindelwald valley is roughly east-west, but the high alps shade the town and much of the lower valley during winter.
The best sunshine is therefore higher up, for instance go up to the First from where you can walk across to the Grosse Scheidegg, and there should be a bus to take you down. Alternatively, from First it is a fairly simple walk to Waldspitz - for refreshment - and then either down to the town, or accross to Bort for the cable car. From the town you can get a bus to Bussalp - though only every hour or so - and walk back down.
Also, take the train to Kleine Scheidegg and walk to Wengen in Lauterbrunnen valley.
Not all of the trails are open in winter. From Maennlichen [cable car from Grindelwald or from Wengen] you can walk to Kleine Scheidegg, but at a rather lower level than in summer - and it's easier and better in this direction.
The trail along the foot of the Eiger north face is closed in winter.
A trip to Jungfraujoch is only worthwhile - because it isn't cheap - if the weather is perfectly clear - there is a webcam, do a Google.
There's been quite a lot of snow this year, and it has been colder recently than normal; and there has been much low cloud - you need to get above about 900 metres to get the sun and see anything.
The tourist centre in Grindelwald is usually helpful, and they have bus and train timetables etc.
Never been to Venice.
Have a good trip.
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