Well, do you want sound? how about lighting functions? If you want sound, you can get a board replacement decoder, or a tsunami that would plug into the 8-pin socket. If you want more than 2 lighting functions, you could get a board replacement decoder, or one that plugs in to the 8-pin socket, but has solder pads for additional functions.
If you don't need either of those, a simple decoder will work, like the DH123P which has the 8-pin plug which can go into the 8-pin socket, or you can take the harness off and plug the decoder itself into the 9-pin socket where the dummy plug is.