The flattest east/west street is Pine street either other direction from that you start getting steeper.
To get from Pike Place Market to the Waterfront you can take the elevator that is back behind the bakery and the BBQ place, it goes down to Western ave, you cross the street and take the elevator in the parking garage all the way down, work you way across the parking lot and then across the street boom it's the waterfront an. I think there is a another elevator, but I have never used it.
If you want an water tour, check out Argosy, they should be wheel chair assailable.
There is a Piroshky Piroshky at the Columbia Tower food court if you don't want to wait in line, my guess is that they are only open during the week and close at 3 or 4pm.
Chiluliy offers a Space Needle combo ticket, that looks to be a good deal if you want to do both.
Pike Place Market (never Pikes Market) is going to be nuts, especially with a wheelchair, so bring your patience
. Make sure you get off the main area and check out Down Bellow and the back halls of the Sanitary Market. Kitty corner from the Pig and fish throwing people is a fruit stand that has the best yogurt, also check out the pickle guy who has free shots of pickle juice, yum... Early morning is kind of magical the coffee (including the original Starbucks) and pastry places are open and you can watch everyone getting set up.
Since your going to Alaska, make sure you check out the Klondike Gold Rush museum run by the National Park Service, the other end is in Skagway, They offer a nice self guided tour of Pioneer Square. FYI - the Gold Rush changed Seattle forever.
Unless you want to leave downtown, I wouldn't rent a car, traffic and parking can be an adventure. Seattle is pretty compact easy to walk w/ good transit and taxis, Uber are not too expensive and we also have ZipCar, Car2go, and whatever the BNW one is.
I would stay downtown over Queen Anne/Space Needle or South Lake Union (SLU), but anywhere that you can get a room, Seattle fills up in the Summer, so book soon.
Bartell Drugs is the local drug store chain, 100+ year and going strong. I always walk out with something I didn't plan to buy. There is a Target and a Kress/IGA (grocery store) right downtown and a Whole Foods in South Lake Union aka
Amazon Land (I think).