I've found that caffeine helps to keep me focused when I don't take medication. Since I'm assuming you don't drink soda, green tea works very well. If you're looking for a light medication to take, I've been prescribed methylphenidate as my mid-day/afternoon light dose, and it works tremendously. I have a 5mg dose, and while they're hard to actually find at pharmacies around me, they should be fairly easy to get prescribed since it's used to treat so many different things. Hope I helped!
Yes, I agree caffeine very well could help.
Strenuous exercise is also very helpful. Studies have shown that the results can carry over for up to 4 hours. Running or any heavy aerobic activity is best for that. Kick boxing may not last long enough at a higher heart rate to help for as long a period of time. Yoga is great for the anxiety that can come with ADHD.
Any reason why you don't want to try a stimulant medication? It might not have been a good combination when you were taking your asthma meds, but would not be a problem now.
Here is a link that provides alternative sources to deal with ADHD. It might be kind of helpful, but there is not much out there that has been proven. http://www.additudemag.com/slideshow/21/slide-1.html
Hope this helps, please post if you have any more questions.