We have finally started to make plans for our April '23 trip to Japan more definite and would love some comments about the few days we'll be on the Izu Peninsula.
We're not great hikers but leisurely walks around beautiful places are always a joy. Mostly we enjoy small towns, interesting shopping, good food and the general beauty of Japan, having visited about 8 times now.
We'll take the train from Nagoya to Atami on 7th April and stay one night there... visit the castle and other sights??
Next morning we'll pick up a car and drive slowly to Shimoda where we will also stay one night. We'd love advice about places to stop along the way - to sight see or eat.
Next day we'll head up the other coast and stay that night in a hostel in Namazu... chosen mainly because it is an easy driving distance to Atami where we will return the car next day, and take the train up to Narita to meet our daughter and grand-daughter arriving on the evening of 11th April.
We'd love any advice about how to make the most of what looks like a beautiful area!
Thank you in advance!

The castle in Atami isn't a historical castle. It was built in 1959 and is essentially a touristy museum about castles in Japan. It can still be fun, but just manage your expectations accordingly! Atami's most famous sight is probably the MOA (Museum of Art), and you might also enjoy the geisha performance which they do on weekends. You can find a schedule here. Kiunkaku is definitely worth a visit, too. With your dates though, you might also want to include some cherry blossom spots. I'm sure they have a sakura tunnel somewhere in Izu.

There's another art museum down there, Ikeda 20 Seiki Museum, but the best stop is likely Jogasaki Coast. There's a bit of a walk along the rocks there. Other rocky coastlines include Dogashima (on the other side of the peninsula) and Cape Irozaki (southern). They do boat tours around Dogashima which will take you to a very photogenic cave. See more info here.

For a different type of walk, you can go to the Kawazu area (Kawazu waterfalls walk), and since you're driving, definitely stop by the Kawazu Nanadaru loop bridge.

Ito for some hot springs on your way down, or Shuzenji onsen on your way up if you prefer, though there are plenty other onsen in the area (ex.Yugashima onsen) that are more isolated and quieter towns. Another town I recommend is Matsuzaki (see here), which is more relaxed than the cities where you're staying the night. You can ride a rickshaw in Matsuzaki, but better email him in advance to decide on a date and time. In towns like these, even if activities are announced, they may not be available if you just show up!

Finally, Shimoda is famous for being one of two ports open to US ships after Commodore Matthew C. Perry's arrival, so you will see many ships, Perry street, etc. around the city.

Good luck! The Izu Peninsula is a nice place to visit!