You can enjoy a good meal and discuss important issues with your mentee. If you're like most of us, you spend a lot of time in meetings. Since you have to attend the meeting anyway, it won't take much additional time to prep your mentee on the way and debrief for a few minutes afterward. Mentees can find these meetings extemely valuable. The help you receive will make up for the time you invest in instructing your mentee.
Projects are a great learning tool.
Your mentee can learn from them as well. Your only time investment will be making a few phone calls to complete introductions. These are all great learning tools. As in the example above, your mentee can spend time learning without your being there. To save even more time, have your mentee research the options.