What science-based strategies can slow down dementia? What are some science-based strategies that could slow down dementia?

For example, do cognitive demanding tasks like solving crosswords, playing with the Rubic's cube, chess, or perhaps stuff like gardening help against dementia?
It depends on the disease that causes dementia. Dementia is a syndrome that has many causes. Some of them are treatable, such as metabolic/infectious diseases. Some are primary degenerative diseases of the nervous system, such as Alzheimer disease, Pick disease, and Huntington disease. Most diseases in the first category are treatable, many are curable. But most diseases in the second category are usually not amenable to treatment. They progress slowly but relentlessly despite any treatment.

Regarding non-pharmacologic treatments, including cognitive training or cognitive rehabilitation, they seem to have some roles in the management of dementia (H C Kales)   but not have spectacular effects and still be inconclusive in improving or maintaining cognitive and functional performance of people with dementia (Diagnosis and Treatment of Dementia).