Downtown is full of sidewalk cafe's, and is probably one of the most active restaurant scenes in the intermountain region. Some are expensive, some more affordable so it just depends on what kind of food you are interested in.
Many of the restaurants have menus posted near their entrances.
Hyde Park near downtown also has some great restaurants/sidewalk dining.
One thing you will notice is downtown Boise is not big on the chains, but is full of what has been called boutique restaurants=more intimate and local and organic!
The river is ready for floating and you can spot people fishing for steelhead and trout right through downtown. The Boise River comes straight out of the mountains and is clear and cold and clean. There is a city/county ran park called Barber Park which is in East Boise and they have tube and raft rentals and there is a city bus that runs from a park downtown to the put in point for the river. Check out these websites for more info:
Boise Convention and Visitors Bureau,
Downtown Boise Association - Welcome to Downtown Boise!.
Floating the river is one of Boise's must do's and one of the most popular activities in the summer.
As far as hotels in the downtown area that you can walk from, try the Hampton Inn at BODO on Capitol Boulevard, there is Hotel 43, The Grove Plaza Hotel, and several others that the website I provided will list.
If you are in Boise on a Saturday be sure to check out the Saturday/Farmers Market downtown on 8th Streets. There are over 100 booths of organic veggies, fruits (the Boise area is a breadbasket in the mountains), wines, art booths, and performing artists too. The market in Boise is one of the largest organic markets in the Western USA and is a must do on a Saturday.
Have fun.