Sounds like advice on the brewery side is pretty well covered. I'd say use an afternoon to go to Fremont Brewing followed by Brouwers (both rather close to Lake Union, on the eastern side of the water/the side opposite downtown). And if you need to include some non-beer focused events to help you negotiate with the GF you can cover gasworks park, the Fremont troll, Theo's chocolate (right across the street from Brouwers and a really good chocolate shop) and the general interesting neighborhood that Fremont is. All of these are walking distance from each other and your GFs aunt is probably reasonably familiar with the area.
If you have the ability to fly some bottles back I'd check out bottleworks. They have a really good selection of local stuff as well as harder to find beer from out of the area. And they have a bunch of taps for drinking there/growler fills as well.
Other options that haven't beer mentioned yet:
The Noble Fir is Ballard is a nice bar that keeps a small but focused list of beer and cider
The Burgundian is a newer bar with the same owners as Brouwers and Bottleworks. It is near Green lake and about a block or two from one of the Elysian locations
If you are also into cider Capitol Cider is relatively new and I haven't been there, but they seem pretty focused on representing local craft cider and have a bit of beer as well.
Have a good trip!