Lake  Lucerne is just beautiful. You can have a ride to Lucerne e.g. from Treib or just go to Brunnen for a drink or have a round trip on the Urnersee. They operate also many old steam boats and you can book evening cruises or e.g. go to the Fondue boat. For more information see the website where you can find the timetable and many other offerings. 

http://www.lakelucerne.ch/en/

Around Beckenried you have a lot of options to have a swim and spend the day in the sun. 

Bauen is a nice little spot to relax and swim. It has a Restaurant and a little kiosk). Bauen can be reached by car (drive trough the Seelisberg tunnel) and on the way to Bauen you can watch the kite and wind surfers in action. You can also walk down from Seelisberg to Bauen (2 hrs) and take the boat back to Treib and up the way with the cable car. 

Seelisberg has a nice little lake with a nature camping site with swimming place and kiosk. Please park the car at the car park or in the village and walk down. 

Entry fee applies.

https://www.myswitzerland.com/en-ch/seelisberger-see.html