[edit] Steps

Play fetch with a tennis ball. You can often get them free from around tennis courts. Some Labs won't bring the ball back to you without being trained to do it.
Get your Lab in the water. They may like boating, chasing fish, and swimming. Know your dog's preferences about water before assuming they will like it or not. You can also get them a little backyard swimming pool or watering trough to dunk in during play.
Give your dog a chance to run. Most labs have bolts of energy so you might want a big chunk of land. Or, you can take them to an off-leash dog-park every couple of days. If there's any water there, bring a towel.
Train your Lab - this breed does very well with training. Enroll in a class together.
Consider agility training for you and your lab. While they may not be a fast dog, they usually do quite well. It will make them mentally alert, eager to please, and fun exercise.

[edit] Tips

Make sure to have a lot of time and attention for your Lab, or else they'll get bored and depressed!
You don't need lots of expensive toys to keep a lab happy. A few tennis balls, some things to chew on (we like cow hooves), and quality time with you is what matters to them.