I parked at the Marriott. If it had been on my dime then I would have parked on the street..... It's quite easy and there were spots across the street when I arrived.

It costs EUR 30 (ouch!) to park at a meter per day in downtown Brussels. I think you can pay up to a half day at time if I recalled the instructions correctly. Hence the EUR 27 fee by the Marriott....... However, the meters are in effect 09:00 to 18:00 on weekdays so not a bad option if you leave in the morning.