We have visited Santorini and Crete. We are returning to Greece and are thinking that we should visit the Ionians, Peloponnese peninsula, Athens and not the Cyclades. We do not want to run around too much.
The only island we've visited near the Peloponnese is Hydra. It was beautiful but rather small and lacked sandy beaches like the ones found in the Cyclades.
Matt Barrett's guide to Greece is very useful and has a link to all the islands in Greece. This might be a good starting point for your planning.
https://www.greecetravel.com/
Here are the possible options if you want to visit the Peloponnese by car and one (or more) Ionian island.
Poros and Zakynthos Islands
From the port of Kyllini. Several ferries per day.
https://maps.app.goo.gl/Qjw5ZjTUDkBRHev7A
Kefalonia and Ithaki
From Patras port: 1 ferry per day
Kythera
From Gythio. 2 to 3 ferries per week
https://maps.app.goo.gl/BuYLVsizQwTCFzLp6
or from Neapoli, 2 ferries per day
https://maps.app.goo.gl/ifu1T8GjcKT18GAo6
Although geographically Kythera does not appear to be in the Ionian Islands group, administratively it is. And it's the easiest to reach if you're going to visit Nafplio / Monemvasia in the east of the peninsula. (we stayed in Kythira for 6 days last year, it was part of a trip to the Peloponnese).
Some photos from Kythira: