Both Sark and Herm are fab. Not much open in Sark these days but have always found food at La Sablonnerie (little Sark) lovely plus gorgeous peaceful setting. Most people head to Stocks on Sark which is nice but I personally wouldnt rate the food.
Herm good - make sure you visit the little chapel there and Shell Beach (if weather good). If it’s a blistering hot day The Mermaid Tavern will be rammed with a great buzz about it - perfect for a cold beer or cider!
Another nice little lunch place in St Peter Port is Cafe Delices (near old market place and opposite New Look), think fabulous french crepes (savoury and sweet) though you can’t book. If you want Chinese then China Red is your place - just a short walk along the Esplanade from town and voted in top 5 Chinese Restaurants in UK I believe. A good Indian is the Taj Mahal (along the front near m&s) - the Patilla Beef is amazing.
Also in town good for a drink/views is The Terrace - not far from bus terminus. It’s outdoor and does nice Thai food too though is self service and very busy on a sunny lunchtime. Also worth a mention are Gusto (Italian near town post office but also do a take out express along the front) and a pub called The Swan (on St Julian’s Avenue bear to the big roundabout) - great burgers in there and pretty kicking on a Friday night.
Craft beer place called Golden Lion just up from Cafe Delices if you like craft beers and poss worth heading up to Copenhagen (by Hotel de Havelet so not far from Octopus) for a drink on their outside terrace if it’s nice as some good views there. Food a bit hit and miss there.
You can also do a Rocquettes cider tour and Annette Henry does walking tours of Guernsey. Also worth doing is one of the puffin rib rides. All should be easy to find on the internet.
Quaint little tea room out by the coast is Cobo Tea Rooms who do fab cakes. And a country pub out in St Saviour who do a good Sunday roast is Fleur du Jardin (spa there also - Lavender rooms). Also great for cake (and food generally) is The Kiln at Oatlands. There’s also Rebecca’s Chocolates there - hand made chocolates on Guernsey.
Not sure when they start doing it but lots of hotels offer, during the warmer months, “dip n dine” so you can use their pools and have a meal - quite lovely really. Some I can think of who do this are Fleur du Jardin, Penninsula, The Farmhouse.
Hope you have a really fabulous time!