Euro Tour - need help!

On the 19th if the weather is clear visit the Jungfraujoch as planned. Depart as early as possible since the clouds gather in the afternoon up there. If the weather is poor or cloudy, then skip the Jungfraujoch and do something else. Here are some poor-weather suggestions:

Brienz has a wonderful open-air museum filled with traditional Swiss buildings, traditional farming techniques, etc. From Interlaken you can take a brief train or boat to Brienz and then a short bus ride to the museum. The museum and the train/bus are covered by the Swiss Pass.

You can rent a bicycle and enjoy the relatively flat area around Interlaken. You could also take the train to Lauterbrunnen and rent your bike there. Riding up and down the flat valley is lots of fun.

The Lauterbrunnen valley also has Trummelbach Falls, which are a fascinating piece of geology - waterfalls inside a mountain. Covered by the S.P.

Lower-altitude hiking trails should be open in April. Check with your hotel or tourist office for a map and advice on exactly which trails are open.

ETA:
The city of Luzern has plenty to explore aside from the nearby mountains. The old town is lots of fun to walk. The Rosengart museum has a collection of Picassos and other modern art. The Swiss Transportation Museum is fun if you love trains and such. Both museums are covered by the S.P. Since you have two days in Luzern, I'd set aside one just for the city and the other day for mountain excusions.

I think you have Amsterdam covered. 3 prime things in 2 days is enough, you'll be tired the first day. Maybe add the Rijksmuseum if that is you interest.

Paris: Get the Navigo Semaine pass for 22.15 EUR plus a 5 EUR initial fee. Bring a 25 by 30 mm passport type photo that will be attached to the pass. That will cover your transport by Metro, RER, and bus for the entire Paris region, including Versailles and Disneyland. It will expire Sunday night, so get the day trips out of the way before that. Monday and Tuesday, you can just buy single trip tickets for less than 2 EUR each.

Have you bought your Thalys and TGV tickets yet? If not, now is the time to get discount nonrefundable tickets. For the train to Interlaken, if you have not already purchased it, take an earlier train. Don't get hung up on the direct train that gets you there after midnight. There are earlier trains at the same price with a single easy connection in Basel.