You can always spend money (either yours or your department's) and buy the TA's something they might like (e.g., dinner or a Starbucks gift card). I would limit the gift to 6 beers on my international beer scale. This, however, is the easy solution. Most TA's, are not doing it solely for the money and while a dinner would be appreciated, it is not nearly as valuable to them as other things you might be able to provide.
A great reference letter than speaks to both the TA's teaching and research abilities can be very valuable if there is enough difference in seniority between the instructor and TA. If there is not a big difference in seniority, you could still recommend the TA to whoever hired you to instruct the class. You could, for example, recommend they be given additional classes to TA or even to be an instructor.
If you are not in a position to write a letter of reference, there are still other things that can be done that are more helpful than dinner. For long-distance TA relationships, inviting the TA to give a research seminar at your university can be a valuable networking tool. Ideally, this would be paid for, at least partially, by your department, but even an unfunded invitation for a PhD student can be useful. You could also arrange to meet at a conference and introduce them to your network. You could also volunteer to read some drafts of their thesis/articles or be a sounding board for proposals.