For certain types of layovers (family, need to stay in/near Bangkok or the airport, 1 ~ 3 days) I have been recommending the Dusit Princess Srinakarin to FTers who send me PMs. It is a nice property, close to the airport and adjacent to the Seacon Square Mall. This mall has the usual assortment of anchor stores, but also has versions of Khlong Tam (toys, electronics, in Chinatown), Pratunam (clothing) and Panthip (IT). Not sure about designer stores though. Also excellent food courts (several), hundreds of restaurants (try MK or Oishi Shabu Buffet) and a ~ 14 screen movie theatre. There is cool vintage motorcycle exhibition there now, and a mobile phone show/expo.

If things have settled down in town you can hop a taxi to On Nut BTS for 70 baht; travel time varies but ~ 20 minutes even with traffic.