There are a few important features to consider when choosing a monocular. First, as you'll be taking it outside plenty, try to find one with good water- and fog-proofing. The cold air exposure after it's been stashed in your bag or pocket will mean fogging up can be an issue.
If you're after a high-tech option that can take photos, you can find models with built-in image capture features, while others come with smartphone mounts or built-in tripod mounts for 'digiscoping'.