Dave - a Barlow or PowerMate (Teleview) will give you magnification increase - they come in a variety of "magnifications" (and prices).
As for filter,I understand (its been a while) that different coloured filters (wratten colours e.g.) or other specialty filters will enhance certain features of the planets depending on what you want enhanced. For example, a blue filter might enhance the polar caps of Mars (that could be completely wrong but I think you get the drift).
Others here will no doubt chime in with other good suggestions - particularly those planetary imagers out there.
  