I think it's pretty hard. There are already some very well-established ones out there (e.g., flyertalk, lonely planet) addressing the "general travel" market. You won't take them down. What you're left with is niches (surfing on beach XYZ, food tours in country ABC, etc.), most of which aren't too large. When you consider how forums monetise much more poorly than other sites, you are not exactly looking at becoming the next King Midas.
Travel is too broad a niche. If you want to make money with Adsense focus on something. And forums take an enormous amount of effort to run. You spend way to much time fighting spam. Shoot for a tightly defined blog, perhaps a blog about hotels in your city.
Blog about Hotels. That would be a good one. Or something financial related. Travel may not make you any money unless you focus on something.
ToBad, unlike others I think like a winner, not a loser. I'm in a process of starting a forum board that will rule the internet in general travel. I got what it takes, a domain name to die for and websites that get more than 1 million unique visitors a month. Few of them are travel related (5000 uniques a month). Wanna join forces to get the travel forum happening? Let me know what you have to offer.
focus with in a city. sample travel name of city. not just focus on the adsense. try to sell some banners and ads on your forums. Try to contact some local hotels, inn and travel guide on the local area for advertising.
There are big competition for every niche that's worth any damn. At the end of the day, you either improve the existing model or you bring something new to the table.