Ideas needed for bases in April- 9 days bookend in Zurich

A couple of suggestions. Kandersteg in the Lauterbrunnen area is delightful and offers spectacular scenery( the Oeschinisee) and lots of hiking.

Something a bit different is Vaduz, Liechtenstein. About 90 minutes by train, although you actually get off the train in Switzerland, it is a quick taxi ride from the station in Sargans. If you want over the top accommodation and food try the Parkhotel Sonnenhof.

Unless you especially wanted to see Zurich, a lot of us have taken advantage of the easy train travel to start or end a trip in Lucerne, which is a short ride away, so that might reduce your bases if flight times allow.

Great idea for your anniversary! I happen to think April is very beautiful! Staying somewhere at a lower elevation can give you the best of both worlds - travel to the mountains for views/ snow/perhaps more winter conditions and enjoy the spring weather/flowers/green grass where you are staying.

No one can predict the weather, but looking at opening/closing dates as well as local events can help you know what the season is usually like at certain locations.

For example Morges, a town on the lake of Geneva, has its annual tulip festival in April. It is a nice time of year to visit this region.

https://www.morges-tourisme.ch/en/Z38861/tulip-festival

Probably just choosing two destinations during your 7 days would be a good amount. You could do Luzern and visit some of the mountains there. Stanserhorn opens April 11th, for example.

https://www.stanserhorn.ch/en/

Since you want to optimise the Alps and scenery, this can be a good starting point - then scenic train to Interlaken area. Depending on when in April you are here you might have the spring blossoms in full swing along this route, a very pretty addition to the already beautiful scenery.

Sylvia Michel has a video of walking along Lake Lungern at this time of year 

https://www.youtube.com/watch?v=R4BdFjS5j-g

Where to stay might just come down to where can you find a room available, as even in the slower seasons things book up quickly.

You could try Interlaken/Unterseen/Matten - these three towns are interconnected, look beautiful in the spring, are not as busy as in summer and give you beautiful mountain views and easy access to the Jungfrau region. 

Brienz or Thun would also be good options. 

I was in Lauterbrunnen last week. It was so beautiful and warm I did not even need my coat as I walked around. No snow, also no snow in the village of Wengen…but again, weather is not that predictable. If you want to do very specific things you will have to check opening/closing dates, and also it might be hard to find accommodation in the Lauterbrunnen/Mürren/Wengen area at this late of a date.

Boats on the Lake of Thun run all year round, but Brienz starts its spring schedule April 4th. Giessbach waterfall is very beautiful in the spring - if you really want to spring for your anniversary you can stay a night in the hotel there.