Most major cities in the world will have a travel guide written about them. If you read many travel guides you may, if you know a particular place very well wonder why they fail to mention that really great restaurant that you know or a fantastic beauty spot. Most travel guides write to a formula and change very slowly, it may actually have been several years since your favorite city was visited.
Post your opinions on sites such as Yahoo! Voices or HubPages and you will probably find a lot of interest in what you have to say.
Give details of places to visit, what to see and do and also post details of little known travel tips, if you know of a local discount card for travelers talk about that too.
Many travelers today are travelling without guides and so the more information you give them the better.
Another tip is that you may also offer to create a personal itinerary for a person, and charge a small fee. Note their interests and supply them with a suggestion list of places to see or eat. Check out sites like fiver.com to set up an account to receive orders.