The Royal Mews was in the second tier of things I wanted to see in 2002, and unfortunately, the top of the Big Bus was as close as I got. I wanted to look at the stables and the horses more than at the state carriages.
The mews can hold up to 30 horses (Windsor Bays and Cleveland Greys)and they are exercised every day. Although the website says that the horses are moved to Windsor when they are not being used for state occasions. When I was in London with my grandson in 2007, they said that the horses were not in the stable, so we didn't attempt to visit there.
The mews is at the side of Buckingham Palace near Buckingham Gate.
The website says:
"Opening times and displays may change at short notice. The Royal Mews are closed during Royal Ascot week in June, and on state occasions"
Open: Aug-Sep: Mon-Thu 10:30-16:30, Oct-Jul: Mon-Thu 12:00-16:00, last entry 30 minutes before closing
Take some time away from all the people around the palace and walk around the lake on the grounds of the palace.
There are many ducks and geese around so you may want to bring some bread to feed the ducks.