The Midway
Last year my wife and I and another couple took the Amtrak down to San Diego and visited the carrier Midway.
From the Amtrak station to the carrier is a short 10-15 minute walk and the visit to the carrier was quite something, especially if you've never been on one before. The trip isn't free as you have the train ticket both ways and the cost of entry into the carrier, but it's well worth the trip. One of the best advantages of taking the train is that it adds an bit of additional excitement AND, the biggest advantage is no parking problems. Trying to find a parking lot close to the carrier would be difficult and expensive. The train trip is approximately 2 hours long each way and you'll spend a couple of hours on board. Many areas are accessable though you have to climb stairs which are quite narrow, but kids would enjoy it. There are guides all over the place keeping you on track and answering questions and many of these guides are individuals who served aboard the Midway during the tour of duty. I wouldn't recommend it for Cub Scouts, but for a scout troop it would be interesting.
We took our Pack up last year and it was great. I would recommend for the Pack or Troop level. We took over thirty boys from Tiger to Webelos. Every age level enjoyed it. They really liked staying up late to watch movies on the ship. I recommend a 1 to 1 ratio for kids to adults so the younger ones aren't scared or homesick, but hey, what parent doesn't want to see an aircraft carrier! :)