When I turned sixteen, (many, many, many years ago), my party was at a very famous and fancy tea room - ladylike and proper, we all were dressed up and loved it. While that is less the trend now, depending on the birthday girl, sometimes I still think simple, but fancy is a real treat for Sweet Sixteen and certainly special. Just do one very nice thing. For example, pick a nice restaurant, but go for lunch instead of dinner because your $ goes farther. I am thinking maybe the Georgian Room at the Fairmont. Perhaps there is a type of food or culture that she favors; ie; french and Cafe Champgne? Or, if you want to venture beyond Seattle, dine above the Falls at the Salish Lodge. There some restaurants that have private dining rooms that can make the entire event feel extra special, and are good for smaller groups.
How about the Queen Mary Tea Room? http://www.queenmarytea.com
Marrakesh is an excellent idea, feels "grown up" but still fun and casual.
Buca di Beppo is another fun option. You can reserve the "Pope" table, it's just the right size for your group.
The Pink Door is another reasonably priced option that has that grown-up feel for a birthday group.
Also on the more casual side Von Trapp's in Capitol Hill has indoor bocce ball, very popular, but you can reserve a court for a group, and 8 people can play on a court. Reasonably priced, but you do have to like brats, sausages, wurst and the like. Very fun. Very cool, and minors are allowed until 10pm. http://www.vontrapps.com
How about doing a "Sleepless in Seattle" tour/theme? You could rent an Electric Boat for $89/hr (2 hr minimum) bring along snacks/food (or pick up Dick's burgers for a retro theme) and weave in and out of the houseboats, then head back home for a viewing of Sleepless in Seattle - perfect for 16 year old romantics. Using the same theme you could "Ride the Ducks", instead, as they offer group rates.
Lucky Strike in Bellevue for bowling in a more grown-up atmosphere. Not cheap, by any means, but it's more of an experience and they can handle all the details.
What about a "beauty blow-out" then lunch or dinner at someplace reasonable to stretch your dollars? Swink is popular, not cheap ($35/blow-out), but pretty fun. They have two locations, downtown and U Village. http://swinkstylebar.com/teen/
Mane is another option.
Hope that helps.
Edited: 9:14 pm, October 23, 2013