My friend who lives in Thun recently took her son swimming in the River Aare in Thun, which is a 30 – 40 minute train ride from Interlaken and slightly longer by bus, so it’s a great day trip from Interlaken.
Schwäbis River Pool in Thun
https://www.interlaken.ch/en/experiences/poi/schwaebis-river-pool
Thun is a lovely city with interesting architecture, castles, parks, wooden sluice bridges over the river, and a beautiful lake front walk. You have great views of the Alps (including Eiger) from the lakeshore. There are nice restaurants along the river and near the swimming area.
There is a beautiful public park, Schadaupark, on the shores of Lake Thun, and a 19th century castle, Schloss Schadau, which includes a miniature steam train.
Oberhofen Castle is further out from Thun and has pleasant gardens as well.
You can also take a cruise on Lake Thun for all or part of the journey from Interlaken.
https://www.interlaken.ch/en/experiences/lakes-waterfalls/lake-thun/cruises-on-lake-thun
There are open air markets on Wednesday and Saturday.
http://www.thun.ch/life/shopping/markets.html?L=1
For the summer, there are several special events that are mostly free of charge through August 14.
“On Thursday evening, Thun City offers free round trips with the trampelworm as part of the evening sale. Start and finish are at the new organic evening market on Waisenhausplatz.
On Friday evening, the twelve bridges of Thun can be discovered with city guides from TTST. The free tour shows the best of the new city tour Thun's Bridges”
https://www.thun.ch/stadtverwaltung/abteilungenaemter/medien/medienmitteilungen/news-behoerden.html?tx_news_pi1[news]=2892&tx_news_pi1[day]=4&tx_news_pi1[month]=7&tx_news_pi1[year]=2022&cHash=28b994841285704ea9316ee05484d1ec&fbclid=IwAR2_p_4qd3qwLbNQYyEmaTzW2NjZsaoe2kK8L9Ycjg5keO_xZ4ohuIKvCaw
See more about the Trampelworm here:
http://www.trampelwurm.ch/
All the best,