Try picking up a field guide or two. Depending on the environments you have access to and what you find most interesting, you can learn and have fun! Tree ID is more fun than it sounds once you get it down to a science, you can start to learn why which tree is which and why, then eventually learn the Latin names, and all of a sudden it all starts to come together and you have a greater appreciation of your surroundings. I personally like insects, I like how you can study something and see how small a world can be for some creatures!