in my post i forgot to address your questions.
walking: no and no. we walk every day as it is, with our dogs. not many people would be happy to join two energetic rotties. as for your chances with others... i don't know about sosua but in POP people walk on the malecon and it is a social occasion. you may be able to create a group but i think people would not likely pay for it as walking outside is generally free
but why not make t-shirts with your logo saying "walking club" or something. you can sell them and make some cash and people who walk alone could join someone if they see their tee.
prepared food: no. i have flexible job and i can cook fresh every day. but lots of folks cannot so maybe they would. there is more choice of health foods in shops but many have no idea how to prepare those. i bought quinoa once and it was nasty. so there were leftovers from lunch. for my dinner i decided to eat that damn quinoa cold, adding cucumbers and onions. it was delicious! so maybe you can prepare those weird foods in different ways so that people can try different dishes.
dietitian and nutritionist: no. i am 125 pounds, i really do not have to change a diet. but miesposo went to endocrinologist for his diet advice. so there is a great chance that people will be interested in solid advice from a specialist that is individually catered to their needs and tastes.
ps, when i say "people" i mean mainly dominicans because i think it is important to direct the business at them as well. this is their country and they live here full time. if dominican middle class is interested and becomes a core of your business you will have clients all year round. just a thought.