If you are just going for 2-3 days, the best place to stay within a budget is this hotel:
Econo Lodge Downtownmore infoAddress:Address:
475 Rideau StreetOttawa, ON K1N 5Z3, Canada
This is in the Main Rideau Street, walkable to/from main downtown and US Embassy. You enjoy walking or you can take a public bus too. It hardly takes 10-15 mins to reach Embassy by walking.
I remember staying to this hotel almost for 10 days while I was there. There are lots of nice places to visit. Mail me if you are interested to know better.