Let's go straight to your questions:
1. Interlaken is not particularly scenic, but fits your itinerary very well because it's centrally located and offers good train connections to all the towns on your list.
2. Chillon castle is quite a long trip from Interlaken, but you can combine the visit with a trip on the southern leg of the Golden Pass line
. Castles much closer are in Spiez
, but only the one in Spiez will be open in April.
3. All hikes are listed here
. The ones below 1000 to 1500 m could be clear of snow, but please check with the tourist office beforehand. Lauterbrunnen to Stechelberg
and Grütschalp to Mürren
(very scenic, there's probably still snow but it will be cleared) should be fine.
4. The Swiss Museum of Transport
is excellent, so you're right to put it on your list. You will get a 50% discount with your Swiss Travel Pass. Other options are the JungfrauPark
(not included) and the Open-Air Museum
(free, but only open from April 11).
5. I'm not sure what you mean. All details about the trip are here: myswissalps.com/rigi
6. Which pass is best depends on your exact itinerary (e.g. whether you go to Chillon Castle or not). Once you know that, you can do the math as explained here
. Print at home will be offered soon, but not I'm not sure when exactly. You don't necessarily need that though, as you can simply get it delivered for free via raileurope.com/rail-tickets-passes/swiss-pass
7. You can buy the other tickets along with your pass on the above website. All you need is tickets from Paris to Basel, and Domodossola (Italy) to Venice. Everything between Basel and Domodossola is included in the pass, so you don't need tickets there. This topic
is useful too.
8. You need Swiss francs, please see here
9. In general, shops will be closed during Good Friday and Easter (not on Saturday though). It varies for restaurants. Tourist attractions are open, but do check before you go. Please note that many cable cars will be closed in April, as it's a transition season between Winter and Summer.
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