I don’t know Jersey but I have visited Guernsey many times.
The south coast is stunning- lots of little sandy bays linked by a coastal paths. You can do a great walk between them, most have a “kiosk”, a little cafe. I really like the Moulin Huet tea rooms. Monet painted a lot here and you can see his paintings of the bay in the tearoom. If you go to a kiosk, make sure you have a Guernsey ice cream, the milk from Guernsey cows is very creamy so the milk (and ice cream, butter, cream) is really good.
The west coast has big open bays and Atlantic rollers. Go to Cobo for fish and chips at sunset.
In both cases, make sure you check the tides- the beaches often totally disappear at high tide and conversely it can be a huge walk out to the sea at low tide. I think the tidal range is one of the biggest in the world. You’ll also want to check the wind direction so you are sheltered from it.
You can take day trips to Herm and Sark. I like the RIB voyages and the cider farm tours too. If you get the chance to eat Herm oysters then make sure you do!
Both islands were occupied by the Nazis and the German Naval Signals museum is really interesting. There are lots of Nazi bunkers and batteries etc dotted around the coast still.
Victor Hugo lived on the island in exile and his house is well worth a visit. It’s actually owned by the city of Paris- you have to book tickets on their website.
St Peter Port town centre is a nice place for a mooch. You can walk down to the sea water pools at La Vallette or out to Castle Cornet- which is worth a visit too.
The buses are pretty good if you base yourself in St Peter Port. They won’t go anywhere near the south coast beaches- but the best way to get to them is on the coastal path anyway.
Double check if there will be a ferry between Jersey and Guernsey this summer- they re-tendered their ferry services, the islands fell out and went with different operators, I’m not sure quite how it all ended.