The cheapest way out is to simply replace the radiator cap, or temporarily switch a comparable cap from another car. Idling it in gear in the driveway serves no constructive purpose. You'd be best off blocking the radiator with a piece of cardboard for around 10 minutes to get it up to temp and pressure.
If you have one of those plastic radiators, they are known to leak around the neck where the cap goes on. If the cap doesn't work, take it somewhere to be pressure checked.
Bob