Guernsey is good; my wife and I went there for our honeymoon. Cars' numberplates (license plates) are just numbers, no letters: you can play "spot the lowest number". We saw 7.
St Peter Port, known simply as "Town" is very picturesque with a harbour and fort. There's a hillclimb held every year on the hill south out of Town, which is the only time the locals get to exceed the speed limit of 25mph in built up areas and 35mph on the open road.
Road junctions are anarchical and work on the principle of courtesy. Post boxes (pillar boxes, mail collection points) are a unique dark blue colour.
If the weather is poor, be sure to visit the candle factory, the brewery, the butterfly farm and the underground hospital from the German occupation in WWII. If the weather is nice, make sure you take in the flat north coast of the island as well as the south side with its steep cliffs - the coast path is good to walk. Try to visit Sark and/or Herm too.