There are many sub-niches under health and fitness. This will all depend on you which one you want to target:
Weightlifting
Aerobics
Hair Loss
Skin Care
Weight Loss
Back and neck pain
Joint pain
Yoga
Cycling
Running
Anti-aging
Eating habits
Eating disorders
Health issues - each by subject - there are so many to list.
The list can go on and on. You just need to figure out which one you want to target and build your site around this subject only. Furthermore, you can break these down into other sub-categories to add these to your website or blog. There are different types of dance people use for health. Many gyms offer certain types of exercise dance classes that a person can join. You have different types of yoga. Right now people across Europe are getting into Beer Yoga. Yes, this sounds very funny but they will do yoga holding a glass of beer. I have no idea who invented this, but for some reason, it is really taking off.
Sub-niches under health and fitness include weight loss, bodybuilding, yoga, meditation, nutrition, mental health, sports training, wellness coaching, and fitness gear.
any Disease, Addiction, Phobia, Or Illness has a niche and multiple subniches. Depression, Stress, Alcoholism, Anxiety, Diabetes, Cancer, Dementia, Eating Disorders. The list is extensive and you can tie in healthy lifestyle tips for all of these niches.