Puppy kongs - you can stuff them with all sorts of things - they bounce around a bit too when the dog drops them so entertaining from that view point as well. I have a round one and long boney looking one and they keep my little fluffyboy entertained for hours. Depending on what breed they are, different dogs like different things - some like to have a wading pool full of water, some like to dig treats out of a sandpit (their own sandpit, not the kiddies' one LOL), some like toys hanging from ropes they can jump at. My old dog was some sort of Beagle X - and she was a real sniffer, so I would sprinkle a handful of cat kibble (because it is very small) over the lawn and she would trundle around happily for an hour or so, finding it all.

Don't underestimate the power of a good raw marrow bone.

The playing and walking is a big part of it though - a good play/walk in the morning and they are more likely to settle down happily in the day when you are not there.