Does anyone have a recommendation for nice moderately priced hotels in Interlaken?
I would recommend not staying in Interlaken, but rather in one of the places further into the mountains in the Jungfrau area. Mürren, Lauterbrunnen, Wengen or Grindelwald.
Info about this area, including hotels here: https://jungfrauregion.swiss/en/summer/
Can you drive to Grindelwald or Wengen? Which are the beautiful towns that you can’t drive to in that area?
Yes and no (in that order). You can drive to Grindelwald and Lauterbrunnen, but not to Mürren or Wengen (train or hiking only).
(Renting a car for 2-3 people is really much more economical than the trains and is much more flexibile time-wise.)
I am not sure about that.
Flexible - trains are at least every hour, often ever 30 minutes. You will need them anyway to get up the mountains.
As for cost, have you factored in fuel and parking, both at wherever you are going to and at your hotels. Do not assume a hotel has parking, always ask. And even if it does it may cost extra.
