Hi!
Guinea pigs have bald areas behind their ears. As long as they have the same shape on each side, there is nothing to worry about. Guinea pigs regulate their body temperature via the blood flow through the ears so hairs that can impede the shedding of heat in that area are not good.
Please have a look at this guide here. You should find it very helpful during your weekly body health check in combination with a health monitoring weigh-in to be able to see a vet promptly with developing illnesses and be able to save the life.
Guinea pig body quirks - What is normal and what not?
Weight - Monitoring and Management
Running (we call it zooming) and popcorning (it is very descriptive for the activity) are very normal for youngsters with boundless energy and joy of life.
More on interactive guinea pig behaviours, boars and some running and popcorning videos in the enrichment guide:
Bonding and Interaction: Illustrated social behaviours and bonding dynamics
A Comprehensive Guide to Guinea Pig Boars
Enrichment Ideas for Guinea Pigs
These guides are all part of our new owners guide collection, which you may hopefully find both very interesting and useful as we have included all the areas we get the most questions and worries by new owners about in combination with some important information to help you minimise the risk of preventable problems in the longer run:
Getting Started - New Owners' Most Helpful Guides