I’m sure you’ll have a breakfast buffet included with your room, that’s usually standard here, but a great place to start your free day would be Crepes and Co, in Sukhumvit Soi 12. It’s been voted Best Brunch in Bangkok several times over the years. It’s not all-you-can-eat, but you won’t walk – or waddle – away hungry, I’ll tell ya! They have lots of different crepes and a heavy emphasis on Moroccan cuisine, but we always go specifically for the brunch, which is actually served all day. You select different items from a menu of choices. Very, very good and filling. Nice house, too. There’s an outdoor area that might be nice this time of year, now that the rains have stopped and it’s become relatively “cool,” but I can’t do without air-con myself and rarely sit outdoors. Best to call and make a reservation beforehand; their English is good.
Hot damn! I enthusiastically second this recommendation, and I can attest that the Moroccan food is good (although I think that might be a promotion they are running now, at least in part-- a couple months back, they were doing Spanish tapas, so there you go). We live around the corner from Crepes and Co and go there on occasion.
Next door to that place is also “Cabbages and Condoms”, which is a restaurant run by the Thai equivalent of Planned Parenthood. The decor is quite amusing, and the food is good if not spectacular. 
(Another good Thai restaurant in the neighborhood that we like is Kinnaree on Soi 8-- it’s our place to take visitors to for a “nice” meal out. I’m sure Siam Sam can recommend many cheaper and more authentic places, though!)
On the subject of brunch, if you want the Brunch to End All Brunches, try the Four Seasons on Sunday. It’s almost $100 a person, but the buffet is to die for: caviar (served from ice pillars in the center of the koi pond), foie gras, Australian lamb, and a full bar. I think the Four Seasons is enjoyable once in a blue moon for the sheer “WTF?” factor, but many of the other hotels have nearly-as-nice spreads (usually on Sundays) for less money. Another thing I have done for fun a couple of times is afternoon tea-- the Sukhothai, very close to your hotel, does a chocolate buffet/tea on weekend afternoons, and the full English tea at the Oriental Hotel on the river (served in the old wing, where W. Somerset Maughm, Conrad, and all the other big honchos hung out) is a fun experience.
The funny thing is Crepes and Co is always running a promotion for Moroccan food and/or Spanish tapas. It’s like a permanent promotion. My wife for some reason receives a regular postcard from them at her office, probably because she dropped her business card in their bowl once, and it’s always a variation on the same thing. The owner is French, I believe. If you’re lucky, you’ll catch a glimpse of his big, fat, lazy cat named Crepe. He’s (the cat, not the owner) a beautiful chocolate color and definitely considers himself master of all he surveys. (Now that I think about it, we’ve not seen the cat for some time; hope he’s still alive!) The wife has tried one of the many non-brunch crepes and liked it, but I’ve never gotten past the brunch menu; for me, it’s simply the only reason to go there, and I can’t imagine trying something else, the brunch is so good.
Another funny thing is all of my recommendations are largely for European- and American-food places. I’m not up on Thai-food places. That’s mainly because I’m married to a Thai. She considers it a horrible waste of money to spend a lot on something we can pick up in the market for the equivalent of a dollar or two. All of our “special” meals involve foreign cuisine, even if it’s just pizza.
But there is one place I’ve always kind of liked, although the wife thinks it ordinary and overpriced (she thinks ALL “expensive” Thai restaurants are overpriced and ordinary, although this one really is not expensive at all) is Baan Khun Phaw (“Dad’s House”), formerly called Baan Khun Mae (“Mom’s House”). Or do I have that backward? Either way, the Mom and Dad referred to are the king and queen, and there are a lot of old photos of royalty. It’s in Siam Square, in the back, away from the main Rama I Road.
There’s also a dinner cruise on the Chao Phraya River. I always enjoy that, and it’s what we usually end up doing with visitors since the wife doesn’t like any other Thai-food places. And now that the rains have stopped, it’s a good time of year. We always book with Yok Yor Marina and Restaurant, specifically the Klongsan branch. Ask for a table topside. Cheap! And a favorite for groups of locals. Expect to hear rousing renditions of “Happy Bert-day.”
I always heard one night in Bangkok, and the world’s your oyster…
Whatever you do, don’t ask the locals what the capital of Thailand is, wait for their answer, and then punch them in the crotch. That really doesn’t go over well.
(On a serious note, I’m bookmarking this for when I make my inevitable trip there sometime hopefully in the next 2-3 years. Thanks everyone!)
That reminds me of an old joke we used to tell way back in junior high school in Texas, about how they chose the king of Thailand. Little did I realize then I’d ever be living here!
Didn’t see it mentioned, but be sure to get a real Thai massage while you’re there (not the soapy sort, although I suppose that can be a bit of fun as well).
For that (the real kind, not the soapy), I would recommend going straight to the source: Wat Pho, or the Temple of the Reclining Budha. On the temple grounds is the kingdom’s oldest massage school. Being licensed from here is a big deal and a sure sign of professional skill.
You’ll see “Traditional Thai massage” advertised everywhere, but you have to be careful, because a lot of these people just hang up a sign and don’t know what the heck they’re doing. This is especially true in the Khao San Road area and other tourist havens. Similarly, in Malaysia I’ve seen “Traditional Malaysian massage” signs, in Indonesia “Traditional Indonesian massage” signs, etc. Some of these people could probably actually do some damage. But at wat Pho, they are highly skilled.
Oh say, Bangkok Bob’s website is worth a look.