Here are a few thoughts:
1. Providing guidelines on expenses for will help a lot for your less experienced employees; and probably some of the veterans, too.
1a. For food you might recommend limits per meal such as $15 breakfast, $20 lunch, and $40 dinner. You could choose higher or lower numbers based on what you and your fellow managers consider reasonable. But whatever you decided, don't make them hard-and-fast limits; allow a person to have an expensive meal on Tuesday after eating cheap on Monday, for example. Make exceptions when there's a good reason, especially if the reason is being in the center of a high-cost city. And, of course, these guidelines are for personal meals and don't apply if an employee is entertaining a business contact. (Ask for written description of the business contact and purpose.)
1b. For lodging you can try setting a dollar limit but it's tricky. As a small company you might find it works better to create a shared understanding about the quality of hotels employees can expect to stay in by suggesting examples as a tier. For example, Courtyard/Hampton Inn/Holiday Inn Express if you're frugal. Marriott/Hilton/Hyatt if you're more spendy. JW/Intercontinental/Four Seasons if you're high end.
2. If work requires an employee to be on the road for two weeks, give them the option of going home for the weekend (if that's feasible) or staying in-city at the company's expense. Roughly speaking the costs are a wash for the company -- a few extra days of hotel and food versus an extra roundtrip of airfare -- but letting employees choose increases their satisfaction.
3. As for the salary implications of spending late hours and weekends on travel duty, consult an HR professional familiar with the laws of your country, state or province, and city. I know that in my home state, for example, the seemingly reasonable concept of "comp time" is broadly treated as a labor law violation by the state authorities. Companies here open themselves to liability if they institute a formal comp time practice.