Places to visit not too far from Istanbul

I'm sure there will be lots of good suggestions, but we were recently in Bursa, and really enjoyed it. It's a two hour ferry ride. We went from Yenikapı terminal in Istanbul to Mudanya, and then took a taxi to Bursa.

We visited to the Ulu Cami (Grand Mosque), the Yeşil Türbe (Green tomb) and the Yeşil Cami (Green Mosque), as well as the Koza Han market. There was a lot more to see (and eat - Bursa is known for it's iskender kebabs) and we would happily have stayed overnight, but it was a fun day trip.

This may sound a bit crazy, but you can do a daytrip to Ephesus by air, if you can get a decent round trip fare to Izmir. I’ve done that and it worked out well, although a very long day! Arranged to have a driver meet me at Izmir airport to take me to site, and then pickup in Selcuk town nearby in late afternoon to return to airport (took local minibus from site to town). This was some years ago when the Istanbul airport was closer to the central city than it is now, but I think it still is doable. Or for a bit more time you could make it an overnight although then you’d have to take an overnight bag and either have to pay for a second hotel room or uproot yourself from your Istanbul lodging.

Another vote for Bursa. The Muradiye Complex (UNESCO-listed) is stunning, and the silk bazaar is fun even if you aren't much of a shopper. This article has some nice photos: https://travsight.com/8-unesco-world-heritage-sites-in-bursa-that-are-worth-visiting/

Bursa isn't on the coast, though it's very close to the Sea of Marmara.

Thank you all for your ideas! Perhaps I will go to Bursa as it sounds interesting and has beautiful sites. I went to Ephesus on the previous trip and it is amazing!
Collette