If you have a Bank of America or Merrill Lynch credit card or debit card, you can take advantage of free entry to 150 museums and other sites during the first full weekend of each month. The program is called Museums on Us and it includes a wide variety of venues, including science museums, art museums, aquariums, botanical gardens, history museums, zoos and childrens museums.
Unfortunately, this offer is supposed to be limited to the named cardholder only. That means that the trip would only be free if you go by yourself or if you find other eligible people who also want to go. However, if you are going to any of these locations anyway, this might be a good way to save a few dollars. Perhaps some of the locations will be generous and admit children with their cardholder parents.
Let me know if you try this offer, if it works as advertised, and if it is worthwhile.