Dogs tend to pick their favorite toys, but like humans, they have the danger of falling into a rut. You can prevent this from happening by getting them new toys and putting their old toys away for a bit. They might sulk for a little while, but then they will start discovering the joys of their new toys. Do this every few months to keep things exciting.
If you are not someone who enjoys DIY, or just don't have the time to fully unleash your creativity, head on down to your local pet store and buy an interactive feeder. These toys require your dog to work for their food and can keep them busy for hours. It is also very fascinating to watch.