Riihivuoren Lomakylä

auto-translated: Pitch right next to a ski lift and a hotel complex.
Also interesting in summer with a great view.
Booked via Campcation.
That's the easiest way. When we arrived, a member of staff was there to explain everything to us. But the booking confirmation from Campcation gives you all the information you need.
Kitchen, toilets and saunas available. Quite nice here.

auto-translated: A car park at the top of a ski lift high above Päijännejärvi, a good 10 km south of Jyväskylä. (The site there, Laajis Urban Caravan, was only bookable online and not for the same day).
Riihivuorentie L. Best to book via campcation. In the confirmation email you will receive all the information including the code for the sanitary facilities. For the last 3 km, do not use the satnav, but follow the signs and drive to the end of the gravel track. The site is large, completely tarmac with lots of buildings around it. Everything was deserted in mid-September, not a soul there.
The easily recognisable sanitary building has everything you need and more: WC, large cosy kitchen, showers, 2 saunas. Tables and benches outside behind the building. But: It's a tarmac car park with a great view of the lake, not a place where you would want to stay longer. OK for an overnight stay.