The Royal Navy Submarine Museum offers a guided tour of one full-size submarine, HMS Alliance, plus exhibits of several small or miniature submarines, including the very early Holland 1 and a well-preserved X24 minature submarine. There are museum galleries depicting submarine operations, with plenty of audio-visual, and a collection of weapons systems control panels as used for launching missiles, plus a Trident rocket.
The admission fee is significant but worth it if you are interested in submarine warfare. If you’ve never been inside a submarine you may be surprised at how little free space there is and how every square foot of bulkhead is covered with pipes, cables, or equipment.
Visit time ~3.5 hours.
Visiting: It might be worth looking at online mapping to see where you are going before setting off (they say a picture is worth 1000 words.) It’s easy to drive past the entrance without spotting it. The site is on the Haslar Road, a few yards past the end of a single-lane bridge that spans part of the harbour, but there is no in-your-face signage, and sat-nav will probably deliver you to an adjacent site, e.g. Fort Blockhouse which is about 100 yards further on.
The site can be reached by ferry from the Historic Dockyard area which is quite close as the crow flies. The ferry seems rather pricey, but the land access alternative goes through miles of congested built-up area.