Most companies that have exempt professionals who travel or work on weekends just inform those people that it's part of the job and that this extra time and effort is one reason why the job pays at the level that it does (i.e., the weekend work/travel is calculated into the pay level of the job). However, giving some kind of compensatory time to exempt employees for their weekend travel and business hours is a good thing to do for morale or, as you put it, “to show appreciation.”
Giving back hour-for-hour is more than you need to do. Many accounting firms have a program whereby for each hour worked by exempt employees in excess of a threshold of say 50 hours during a standard work week, one-half hour is deposited to a compensatory time bank. You might want to create a compensatory time bank for exempt employees who regularly work on weekends for travel and business. If so, you should establish clear guidelines to prevent misunderstandings or misuse. For example, you might want to exclude hours worked by employees who infrequently travel or work on weekends (i.e., have a minimum of some kind that makes sense for you and your business). You might want to exclude travel work hours on Friday evenings or Sunday evenings from consideration, because this a typical travel time for many workers who travel for business. These are just some thoughts on the subject. I think overall that it's a nice idea for morale and appreciation purposes.