Hi mommamenges -
The Cograil train from Lauterbrunnen to Wengen and onward to Kleine Scheidegg ( side branch up to Jungfraujoch) thence to Grindelwald does run all year, and the cableway access to Schilthorn takes its major break in late Fall, so those would both be enjoyable and scenic.
In 2017, the cableway from Wengen to Männlichen began Summer Season service on May 20th, which s typical. It took its mid season break from April 2 until May 19th. Other services begin in late May or early June. By late June, pretty much everything is open.
The timetable for next year will be available soon. ( I think I saw October 18th...) to check the dates for 2018. They don't change much from year to year, but, if the season is unusually warm, or the snow up top is still good for skiing, the cable ways may be open on some earlier weekends.)
At that time of year, Spring is well along at lower altitudes. Although it lacks the majestic scenery of the Alpine regions ( except from a distance) I personally like the Emmental, which is farm country with some nice inns in the villages. I have attached an image from Affoltern in Emmental. It is not a usual tourist region unless you have special reasons for being there. I like to walk in the hills.
In April 2014 there was a very unusual stretch of warm dry weather without showers and with clear sunny days for more than a week in April throughout much of Central Switzerland. Usually, it rains a lot. I happened to be ion the Emmental then, and on April 16th (plus or minus a day or two) took a series of pictures of the hills and mountains and Schloss Trachselwald, from above Sumiswald (altitude about 700 meters) from a ridge above the town, and made a panorama. You can see that there is plenty of snow on the Alps, the grass is greening up in the lowest areas, and the trees are flowering. Spring is just beginning. By May, it is in full swing. In contrast, in the high Alps at 2000 meters or so, the snow is almost always not so good for skiing in April, but the high trails are still closed. Lauterbrunnen at 800 meters is deep in a valley and does not get a whole lot of sun, but the snow on the valley floor at around will be substantially melted, and will be pretty much gone by May. Maybe sooner; the climate is warmer these days.
Luzern (local German spelling) and the surrounding lake and mountains offer some good mountain scenery and is a nice town in its own right. There are close by mountains and also good hiking.
It is definitely not a region with the overwhelming collection of many really high (and snow covered) mountains in plenty, as is the Jungfrau region. But, it is beautiful. But. the Jungfrau region can be reached in a rather long day trip. Too long for me, but many others have done it and been satisfied.
I have attached a few random pictures from near Luzern.