With cooler weather forecast for the weekend, I suggest a day in the Jura, visiting an underground WWII fort near Vallorbe in the morning and hiking in the afternoon.
https://www.pre-giroud.ch/
Cost is 15 CHF.
After the visit we’ll take the train up to Le Pont between the Joux and Brenet lakes, and after a restaurant lunch we’ll hike 10km back down to Vallorbe. At the halfway point to Vallorbe we have the option to visit Juraparc to see its wolves and bears.
http://www.juraparc.ch/fr/index.php
Meeting point is the Fort de Pré-Giroud at 10.00, ie in good time to take the first visit at 10:30.
Finish is at Vallorbe station where we will leave the cars after visiting the fort.
Walking distance about 10km with about 150m height gain and 400m height loss. Hiking time about 3 hours. Path may be a bit steep in places. Please bring suitable hiking gear and something to drink.
I will confirm the travel details, including car sharing, later to participants, so send me a message if you wish to join us. Limited group size for one lunch table.
The hike will be cancelled or rescheduled if there's a risk of bad weather.
Any questions, please ask!

Photo “Canon de campagne Bofors ... au Fort de Pré-Giroud” (c) Clément Bucco Lechat