NOTHING! Do not under any circumstances remove the radiators. There is nothing better than a cast iron radiator for heating efficiency and comfort. Anything you replace them with will not nearly deliver the same value for your home. You can add radiator covers, but sometimes those are worse. They look clunky and boxy and take up more room.
Under the old adage, if you can't beat 'em, join 'em - you might want to hightlight the radiators. In my home, I painted them a contrasting color, which makes them stand out and they look really great.
Just ask our dog Spot, who likes to park himself right in front of the one in our office!
The only other comparable option would be hot water baseboard, but it will not deliver the heat you are used to and will not continue radiate after you turn down the heat.