When you have cleared the last area before exit the underdark (after clearing the Kuo-ta dungeon) just give to Imoen all the drow stuff you want to keep (equip her with a strenght enhancing item) and remove her from the group telling her to wait you in the Copper Coronet.
She will leave and will be able to find her way to the surface (other characters will just remain in the underdark).
You can rejoin her in the Copper Coronet and she will have all your drow stuff (Plates +5, elven chains, xbows of speed +3 etc.) and they want be transformed into dust!!!
Very nice