The island of Lero is a perfect one-day trip from Kos.
Very worthwhile: The two bays of Aiya Marina (east) and Panteli (west) are only about 1km apart, and the town is located on a narrow strip of land that connects the southern part of the island with the northern part.
The houses are mostly well-preserved or renovated, often painted in the typical blue and white. Steep climbs to the left and right, along paths and long staircases. On the southern tip, you reach the Panteli fortress at about 250m above sea level. With great views, including both harbors and the five windmills halfway up.
The towns exude charm and tranquility. This also applies to the people we spoke to along the way, in shops, and in the restaurant.
In the port of Lakka, you walk directly from the road onto a fine pebble beach, from which you can enjoy a shallow, blue water bath.
In my opinion, Leros is definitely a recommendation.
Both - and at least - for a day trip and for a longer stay if you want to totally relax.