BOATS
-Lake Thun is great for castle stops. Brienz is great for water color, optional waterfall hike (Giessbach) and exploring Brienz and/or Ballenberg Museum
-Yes you can get on/off. Check their schedule online. We never do roundtrip. Always one way and train back. If you decide to do it on your travel day, you can technically take your luggage with you and just decide to not get off the boat and enjoy the sites from the water. Then when you disembark you have your luggage and you can continue on with your train ride to Wengen. If you decide to stay on the boat, you can see the castles of Spiez and Oberhofen from the water without getting off the boat. If you do this, "splurge" for first class where you get to sit on couches on the roof of the boat. Most comfortable and best views. It's not that much more expensive and would give you the boat ride experience without taking up a bunch of time getting on/off. If you do this on your travel day that's probably what I would recommend. Stops would probably be a lot to add in, unless you do a full day just for the boat.
-Castle village stops we love are Spiez, Oberhofen and Thun.
-If I had to pick only one, I’d go based on whether you care about the castles or not. If you’re somewhat interested in them but not super excited, then go for Brienz for the water color. If the castles seem exciting to you, go for that. We always get off at Oberhofen and hang out in the park beside the castle. You can get in the water there if you feel like a swim with the swans.
SCHILTHORN
-Yes, the lunch is touristy. But I guess that’s kind of the point, right? We personally just ordered a hot chocolate to experience the revolving restaurant for 15 minutes and then moved on, but not bad to do lunch if you want to. I would sort of doubt the food is ground breaking, but obviously that’s not really the point.
FIRST
-You don’t have to make reservations, but there will be lines. I've heard talk lately of 2 hour long lines for activities. I can't verify personally, but that's what I've heard and it's not even high season yet. It’s very touristy and very busy. Not to say you shouldn’t do it, but be prepared for lines. We’ve never done reservations so I can’t speak to how that goes or if you should/shouldn’t.
VIA FERRATA
-If you’re scared of heights….you’re gonna feel it :) But you’re strapped in so it’s mental, not physically dangerous.
ETC
-If you decide you’re not feeling the touristy vibe of First, one of our very favorite activities is doing the North Face hike counter clockwise from the Allmendhubel (from Murren). There are multiple mountain huts you walk by where you can stop for lunch or a drink. It’s a stunning hike that’s not super difficult. The view from the restaurant up at Allmendhubel is also lovely.