Man, that's a tough one five days before the festival, especially since downtown hotels are already packed with the regular crush of tourists we get this time of year.
Honestly, you might want to stick with the one by Midway if you can get to the Orange line of the El (which ends at Midway) from there. That train will get you downtown in a reasonable amount of time (30 minutes, tops) and you can walk to Grant Park from the Library exit. Check the maps here.
http://www.transitchicago.com/
Or check out this Days Inn a few miles north of downtown. Not a great place to stay, but it's a great location and close to the El.
http://www.daysinn.com/DaysInn/contr...false&tab=tab1