There are multiple small commercial areas scattered around the city so you’ve definitely got a ton of options available to you. Check out SE Hawthorne from 12th to around 38th for a lot of the stuff you’ve probably heard about (second-hand stores, food-carts, bookstores). There is a Fred Meyer at SE Hawthorne and 38th that you can grab groceries at.
Another good one would be Alberta from 12th to around 30th for food/drink/shopping. It’s pretty much the same idea as Hawthorne, but a different set of small/medium businesses.
I personally live in the Buckman neighborhood near Burnside/20th, but the attractions in my area are more focused towards adults (lots of great bars and restaurants). If you do end up in here during the day I would recommend checking out Laurelhurst Park at Ankeny/SE Cesar Chavez. It has a great pond and ample walking trails. Truly brings the forest into the city. Alternatively, you could check out the West side of the river and do some day trips to Forest Park. If you end up getting tired of the city Sauvie Island to the Northwest can’t be beat for a day-trip.
Not sure if you already have accommodations lined up, but you’d get the best picture of the city if you spent a few days in each of the neighborhoods listed above. The easiest way to accomplish that is to go through AirBnB and then rotate to another property after a few days.