Wat Ratchabopit, Bangkok
I only discovered this temple due to an attempted con - it started out with some guy on the street (as ever) telling me about this place which was only open once a year....
Fortunately for me, the temple is an attractive place, and was well worth a wander around. Of course this temple is open daily, but it's less well visited than many of the others; the touts rely on you never having heard of it.
Wat Ratchabophit is on Thanon Fuang Nakhon, east of the Grand Palace, and is open from 5am(!) to 8pm daily. It's free to enter the temple grounds. An interesting feature is the cemetery for members of King Rama V's family (pictured).
The second part of the con, incidentally, is where my new "friend" advised me of this "once a year government sale". Fortunately, I'd heard this bit before, so having had a look around the temple, I made my excuses and left...
Wat Ratchabopit is one of the Wats that Royal People come to visit and pay a prayer to this temple, when we visit this today it is open and tourist is allowed to get inside normaly, they are only open for 3 days in a year so whe're lucky enough to be there though we're late and never saw some of the royal families.
Another great advantage of just wandering the streets and away from the crowds is stumbling across temples no other visitors that day. This lovely Wat was quite ornate and we had it all to ourselves