Rueil-Malmaison is an easy day trip from central Paris. The RER A runs to Rueil-Malmaison station from Châtelet-Les Halles in about 20 minutes (direction Saint-Germain-en-Laye). From the station, it's a flat 20-minute walk to the Château de Malmaison, or you can take bus 258 which runs from La Défense via the château.
By car, Rueil-Malmaison is off the A86 motorway, about 15 km west of central Paris. Parking is available near the château but can fill up on weekends.
If you're combining Malmaison with the Île des Impressionnistes in Chatou, take the RER A one stop further to Chatou-Croissy. The two sites are about 2.5 km apart on foot, connected by a pleasant walk along the Seine.