Two of my favorites are both about five blocks away, and another option is only a block from the hotel.
David Burke's Primehouse is four blocks north and one block west from the Hard Rock Hotel. They feature an "American dim sum" brunch - basically an all-you-can-eat brunch served at your table (not a buffet) in 7-8 courses consisting of 20-30 dishes served in small portions. Not inexpensive at $35 but an excellent value considering the top quality and unlimited seconds. I posted a VERY detailed report the last time I had brunch there, at http://chowhound.chow.com/topics/364403 You can view the American dim sum brunch menu on the restaurant's website at www.brguestrestaurants.com/restaurants/primehouse/index.php
The other place I love is Atwood Cafe, in the Hotel Burnham, two blocks west and three blocks south from the Hard Rock Hotel. They feature an a la carte menu of yummy contemporary American food, very upscale service consistent with a luxury hotel setting, and ultracomfy overstuffed seating. www.atwoodcafe.com
If you want something closer, South Water Kitchen is only a block from the Hard Rock Hotel, and features American comfort food. I haven't been there for brunch so I can't comment on the food/service/etc. www.southwaterkitchen.com
I strongly recommend making a reservation in advance, as they can book up. All three of these places accept reservations on opentable.com (as well as by calling them directly, of course).