Some days will be too cold or too hot for outdoor activities, but mentors and mentees still can learn and play online as well as prepare for outside activities when the weather is clement.
A perennial favorite for everyone is National Geographic--for kids or adults!
For high school:
From the Massachusetts Institute of Technology (MIT), more challenging but perhaps appropriate for high school--or for mentors in need of a serious challenge: http://ocw.mit.edu/index.htm
"Check out videos of competitions at MIT like the Solar Decathlon. Ever see a bullet smash through a rose dipped in nitrogen? Check out the Strobe Project Laboratory. Studying for the Physics AP exam? Watch an MIT professor explain pendulums by swinging across his classroom."
Back to younger age groups:
Education and training opportunities from the U.S. Center for Citizen Diplomacy vary; a few are below. Some websites cost money to join, but most are free. Typically, interactive websites offered by museums and zoos are highly informative and produced well. This may lead you to investigate other museum and zoo sites.
Subcategory: Science, Technology, Engineering and Math (STEM)
The United Nations Cyberschoolbus allows choice of country groupings and then different classifications such as economically developing, Nordic Council, etc.
The Boston Museum's "Ancient Egypt: Science & Technology"
Among other tools, online you can plan your own afterlife. The pharoah allows a certain amount of money; when you search for possible tombs, embalming, etc., you learn the different types and the costs. Of course, you are keeping a budget online, too, and you cannot charge it!
The British Museum: "Ancient Egypt"
From the Globe Program: http://www.globe.gov/teaching-and-learning/e-training
Example: Observing, Describing and Identifying Clouds
"GoNorth!" focuses on the Artic.
"In Search of the Ways of Knowing Trail"
Virtual trip through the African rain forest