There are many places that you can visit on the Island either just by walking around the fortifications on public land or by visiting the museums. There are also excellent guided tours which often give access to sites on private property.
I will be adding to this page but for now a good start for any visitor would be the following links.
Guernsey Museums covers multiple sites
The German Occupation Museum Here you can find my brief review of the museum, details of opening times and some photographs to give you a flavour of the museum. A must visit.
10.5cm Casemate Fort Hommet. Here you can find my brief review, details of opening times and some photographs to give you a flavour of this restored bunker.
Wiederstandsnest Lowenfels (Burton Battery) This site is free to visit and guves great views of the west coast. Follow the link for more information.