The 520 floating bridge trail, while not particularly scenic, is new, wide, and a pleasure to ride as far as commuting trails go. You also get some nice views across Lake Washington. You've found a good route overall, IMHO, and is basically one I've ridden with variations for years—minus the newly opened floating bridge trail. I second woodway's comment that the north loop you've chosen is a lovely ride. The Burke-Gilman trail is one of the nation's oldest rails-to-trails conversions, so there's a bit of history there for the ride as well. And since it's an old railroad line, the ride is very flat—rare in this corner of the country.

Depending on your fitness level and how used to hilly terrain you are, the ride through Interlaken Park at the end may get kind of rough since you're climbing all those switchbacks on Capitol Hill. Alternatively, immediately after crossing the 520 bridge, you can hang a short right, hop on the light rail to downtown, and ride back north from there to your staring point across mostly flat terrain. Bikes are allowed on light rail and there are hooks to hang them. It's a five minute trip or so downtown, and punches right through Capitol Hill, saving you all that climbing at the end of a long ride.

Oh, and be sure to stop in Gas Works Park on your way by it. It's got some fantastic views of the city out across Lake Union.