Airtight User Account on Macbook Pro
The only way to avoid a user being able to access your data is to remove your data from the machine. This is due to the fact that a user could enable the root user via Single User mode on boot and can then have unfettered access to the system. You can put up blocks to hinder this as well, but that depends on your desire to keep all information secure.

A standard user account is the easiest option and is somewhat effective. However, your friend will not be able to do certain things on the machine since they won't have admin rights. The level of access necessary would hinge on how much the friend would need to do while they have the machine.