If you are looking to do an outlet center, I'd try the Camarillo Premium Outlets. Camarillo can be reached by Amtrak or Metrolink from Union Station in Downtown L.A. and various other stations every day of the year and the outlet center is absolutely huge, with a lot of premium brands. The Camarillo train station is about 1-2 miles from the outlet center and a taxi can be called. They are used to this. There are several moderately priced hotel/motels in Camarillo.

As for the Citadel, it is ok but not in the greatest area (Commerce) and not all that accessible by public transport. You could take the Metro Rail Gold Line to the end of the line in East L.A. and get a cab if you can find one, but the area isn't the greatest. Commerce gets very limited weekday Metrolink service, but you would still have to grab a cab. Check out where the Commerce Casino's shuttles go from, because it is located right next to the Citadel. In fact, there is a Crowne Plaza at the Commerce Casino if you are interested, but it is a rather crap part of town as I mentioned before.

BTW, the Santa Monica Place mall is still closed for renovations, but it is right next to the legendary Third Street Promenade. Neither place, however, is an outlet center but rather a regular shopping area.