My dogs aren't too interested in kongs, either, unless there is food in there. Peanut butter works pretty well for it though because it takes a while for them to get it all out, and Adele especially will keep working at it until every speck is gone.
Does Roscoe like stuffed toys? Marta really likes playing with stuffed toys with a squeaker, even by herself. She just loves the feel, and the sound it makes. She has a cool Booda octopus that has squeaky tentacles and rattles, too that she really likes (she keeps it away from Adele though ). Like Ginger's Mom suggested, maybe a feeder toy? They make some really neat looking ones, but I've never tried one.