Visit to the Prado museum in Madrid [limited time free admission]
Unless you have a very intense love of renaissance art and don't have to visit the Prado as part of a group, the museum offers free admission the final two hours of its day
On Saturday, when we went at 1100, we saw the sign for free admission 1800-2000 that day.
Unfortunately we had a day trip to Segovia to see the Roman ruins (an excellent day trip, btw), so we couldn't use the free admission.
We like arts (but we rarely linger in museums), but when you add the crowds at this prime visiting hour, the Prado experience is kinda of messy.
Just trying to push through crowds to see what I surmised to be the same Goya and Goya-esque painting over and over again, I wish we would've planned for an end-of-day visit.
Nonetheless, if you stand the wait, I'd recommend the free admission time. You can definitely do the entire collection in about an hour or so.
Plus, the restaurant you probably want for dinner, won't be open until 2000 anyway. :-)