Plan lessons where you are a participant in the activity and use your participation as modeling. For example, if you plan a lesson on the concept of lift by having students create paper airplanes and have a contest to see who can throw them farthest, you could participate in making and throwing a plane. With an activity like this, you will be able to participate without discouraging students by performing at a level they could not, since the winner of the paper airplane toss will be determined mostly by luck.
Create activities that involve your hobbies and interests. These activities should use your interests as a modeling tool or a springboard to teach general concepts, but they should also let you share the things you feel passionate about with your class. For example, if you are a fan of comic books, you could use copied pages from an age-appropriate comic to teach grammar by having students label and circle all the nouns and verbs on the page.
Play with your students during recess from time to time --- though this should not be done when you are the only adult on the playground, since you will need to act as a supervisor first and foremost in this situation. However, if you have backup from other teachers or playground monitors, your students will not lose respect for you if you shoot a few hoops or play four square --- on the contrary, it will make you seem more human and likeable. It is important, though, to avoid games where your height or size would give you a significant advantage, which might cause students to think you are bullying them instead of trying to have fun with them.
Use humor when talking to your class. It is often said that, given how much time teachers spend in front of a silent audience, they might as well be performers. As a performer, it is only natural to tell a joke here and there. Adding a bit of humor to a lecture or a lesson will not only reduce your own level of stress, it will help students relax and put them in a better mood for learning. If you have trouble incorporating it into your lesson, don't be afraid to simply ask, "Would you all like to hear a joke?" Chances are you will get a resounding yes and will be rewarded with plenty of laughter, even if you aren't sure your joke was all that funny.