Sniffing is work
A dog's olfactory system takes up an enormous share of its attention and its brain. Letting a dog use it properly is cognitively demanding in a way that trotting alongside a bicycle is not.
This is why owners are so often surprised by a snuffle mat. The dog appears to be doing very little, and then sleeps for three hours.
Feed the dog through its nose
There is nothing sacred about a bowl. Scatter the morning meal in the garden, hide it around a room, or work it into a mat, and the same food buys you twenty minutes of occupied dog.
For scent-led breeds this is not a luxury. A Beagle or a Bloodhound with no sanctioned outlet for its nose will find an unsanctioned one, usually involving your bins.
Keep raising the bar
Herding and poodle-type dogs solve a puzzle once and are done with it. Rotate the puzzles, raise the difficulty, and hide things in harder places. Enrichment that has become easy is no longer enrichment.
This is general guidance, not veterinary advice. If your dog is showing fear, aggression, or a sudden change in behaviour, speak to your vet before starting a training plan.