Usually in these types of situations, they serve some type of fruit option for breakfast, so go for that. Be careful of even the muffins and bagels, they can easily have 300+ calories each before you add anything else. Stay away from fruit juice, tea or coffee are calorie-free and you don't know what the rest of the day holds (calorie wise). Have your own stash of Splenda or other no-calorie sweetener if you use it so you are not at the mercy of anyone else.
For lunch, start your meal with a salad or broth based soup if possible, fill up on things that don't have many calories! If you can, bring your own light salad dressing, Kraft has single servings of light dressing available at the grocery store now. If you are having sandwiches, load up on the veggies and stay away from mayo based chicken/tuna salad sandwiches and cheese.
For dinner, again, try to start with a salad or soup. I would be surprised if they did not have some type of healthy option at dinner, such as baked/roasted chicken, fish, etc.
If you just have to have desert or feel pressured to do so, ask for a box right away so you can "save half for tonight". As soon as you get out of there, ditch the box!
Most hotels now have nice fitness rooms, so bring your workout gear and put some time in! I stayed at a Marriott in the spring and they had really nice ellipticals with individual TV's. You can also hit the pool...once I was stuck at a hotel for a week without a car, all I did was eat out and swim at the pool, and I actually lost weight!
Good luck and let us know how it goes
