Bagerhat, a small town, is less than an hour's drive from Khulna, about two hours from Jessore. It was founded by the great sufi mystic and warrior Khan Jahan Ali. There are several old mosques in the area, the most famous being the Shait Gombuj Mosque (Mosque of Sixty Domes.) That is a misnomer - it actually has 77 domes, supported by 60 columns inside. There are several other mosques (9-dome mosque, 10-dome mosque, Shingair mosque, and Ronbijoypur mosque which has the largest dome of any mosque in Bangladesh.) The shrine of Khan Jahan Ali is also in this town. Definitely worth a visit.
I loved the inside of the Shait Gombuj which dates from 1450ish. Here are three photos.