However, for many attractions, you need to take another look at the 2-for-1 rail offers. These offers are not just valid for the day on which a ticket is issued. They're available throughout the validity of the ticket. This is from the FAQs on National Rail's 'Days Out' website:
"If you have a valid rail ticket that is valid for more than one day e.g. Saver return to London valid for one month on the return journey, you can use this to visit an attraction within that same one month period of return travel validity."
http://www.daysoutguide.co.uk/faq.aspx
The list of 2-for-1 offers, with this promotion, starts here:
http://www.daysoutguide.co.uk/Attractions.aspx ?WithOffers2=True
(That's just the first page out of 9. Click the link at the bottom of the page to see more offers).
The current rail 2-for-1 offer is shown as finishing on 3rd June. However, I used to run a railway station and I know that as soon as one offer period ended, another one would start. (So, if you're travelling later in the year, you should still find similar offers on that website).
Chris