I have a super active 15 month old boy too, so I understand how tiring they can be! It would depend on why you don't have energy. It seems like you might have more energy if you started eating better. Foods with high fat and sugar content make anyone feel sluggish. Maybe you could start there and see if that gave you more energy. I'm also a firm believer that energy creates more energy. Even if I'm tired, sometimes I'll put on some fun, upbeat music, grab my son, and we'll dance all around the house. It makes both of us feel good and it's great exercise without even leaving the house. Also, fresh air seems to really perk me up as well. Take a 15 minute walk. That shouldn't be enough to wear you down but could give you some more oxygen, burn some calories and give you some extra energy. Yoga is also great for increasing energy. Especially back bending. This is as simple as standing up, putting your hands on your lower back, and gently arching your back while looking up towards the sky. You can also lie on your belly and slowly push yourself up with your hands to create the same effect.
But truly, I think that diet is the place to start here. Go to the grocery store and get lots of fruits and veggies. Grab those and a glass of water or herbal tea when you want to munch. Get some whole grains and have oatmeal with fruit for breakfast. Avoid anything with a lot of fat or sugar. Take a quality mulitvitamin (and consider getting your iron level checked by your doctor).
I hope you're feeling more energetic soon!