Ear buds are great for listening to music while pursuing your favorite outdoor activity, but there are a lot to choose from and various features work better in different situations.
The first thing to consider is waterproofing. Will you be participating in an activity around water? Even if it is swimming it is possible to get ear buds that you can listen to while in the water. They are more expensive, however, so that is definitely something to consider. Waterproof earbuds will only work if the device you are listening to has been waterproofed as well. Waterproof cases are available for that.
When hiking or mountain biking you may need to hear what is going on around you. Some ear buds block out all external sound while others let some through. Be sure and check because it can be dangerous if you can’t hear in certain situations. In fact, you might want to consider some outdoor speakers instead just to be sure.