Three behaviors that could impact gut microbiome could be:
Diet: changing diet to increase healthy microbes in the gut has no negative effects, unless it conflicts with another eating restriction (i.e. eating yogurt helps increase beneficial microflora, but someone who is lactose intolerant or vegan cannot eat yogurt).
Medication use: Antibiotics decrease gut microflora, but not taking antibiotics could be life threatening if they are being used to treat an infection.
Physical activity: increasing physical activity has very few potential negative outcomes. Being physically active can increase diversity of gut microbes as well as help decrease the risk of things like heart disease.