I came out to Spain 10 years ago, having set up & run small to medium sized business, since I was teenager - 20 years of mainly self employment, developed & sold 7 business.
I am also a qualified fitness instructor & spent 15 further years martial arts / kung fu training, qualified financial services advisor (taken out of boredom) and a few general business studies exams thrown in to boot. All of which you would have thought would give me a good starting edge in a strange land.
When I came to Spain, financially was in a strong position & looking for the lifestyle job - more one in health / fitness, than using my business experience.
As Nigel has stated well below - you have to look very carefully at your local market . Many expatriates are of a certain age & if you believe people come to Spain for a healthy lifestyle, it is not true. There are gyms in expatriate areas, where there is a job or two for a few hours a week, but little likely to be much else. Private tuition, many simply do not have the money. In better off areas - Costa Blanca North or Marbella, but even those areas there will be much competition.
My partner also worked in the health industry offering a sucessful track record of stopping people smoking -she used a device called a bioresonance machine. As this worked well in the UK, and many people smoke in Spain - it seemed like a good idea to bring this along. Whilst she did do a few treatments, the cost of the advertising in newspapers locally outweighs the returns in terms of reader ship - many other small businesses have this problem, the numbers simply do not add up.
Overall the highest expatriate population is Costa Blanca South (much more so than Campasol, which is densely populated, but a much smaller area overall).
I now work for a small telecommunications firm installing TV, Satellite & fibre networks for commnities & individuals - been doing this for in excess of 5 years, through the recession. It is hard work, but we keep our heads above the water.
The fitness option for me now (and being over 40) seems like a distant dream - I know have to keep my business head on at all times, work very hard to keep myself & those around me employed.
Pick something that is either not 100% reliant on Spain (as other poster mentions, export, internet etc) - as unless you have an absoluted gem of an idea, you will struggle. If you keep your expenses to a minimum & don't expect the world, it goes a long way.
The fact is, if you work hard enough & you are determined you can succeed to a given extent anywhere - but be prepared to have to work very hard to acheive this.
Don't dream of getting any kind of useful help from the authorities or twon halls as they in the main useless beyond belief. Even working with a Spanish business partner (to assist with licenses & negotiations), proved a tortuous exercise. If considering a smaller business, you should be (mostly) able to avoid this.