Maybe take to hiding away some of the toys at a time, to vary the environment some, and make it interesting for them. Do other "environment enrichment" things like, one day, leave some paper grocery bags standing on the floor, then some other day leave a couple cardboard boxes, maybe with holes cut in them for them to explore, etc. In short, keep things changing enough so their minds are occupied, and they will find less "trouble" to get into.

Oh, and if they like catnip toys, keep a container - a coffee can or cookie jar will do - with some loose catnip in it, and throw some toys in there to "marinate" for a week or so - it makes old toys new again!