No...seriously...take a hike. Or a walk, a jog, a run, a swim. Rain, snow or sunshine. Whatever you're able to do. Everyday would be great, but every other day is better than never.
A tired dog is a GOOD dog. Or at least, a sleeping dog is a dog who is not chewing things he shouldn't be. He isn't jumping up on people or peeing in the house either. So, strap your big-boy/girl boots on and get your dog some exercise.
If you're currently dealing with some behavioral issues this is my first and foremost recommendation. Get your dog tired. If you have a brand new puppy and you're wondering where to start with training - hey, you guessed it! Get that puppy tired. The more vigorous your exercise the shorter amount of time you'll need to spend to get your friend tired. But it should be low impact and not hard on their joints. Address your training for the behavioral issue after your dog has worked it's bugs out with some exercise before each session.
You've heard of Ceasar Millian right? Handsome Hispanic dude, loves dogs, works to stop abuse and mills, has a TV show. Etc. Well, he calls this step his first of three ingredients to a happy, balanced dog. Exercise. He recommends at least walking and mastering leash manners.
The amount of exercise and how you exercise will depend on you, your schedule, and your dog. For us we take our whole pack up the mountain behind our property for a hike every day for an hour or so. Working on leash training with your dog is highly valuable if you frequent areas where leashes are required or you haven't mastered a flawless recall. But, our hikes are off leash. If your goal is off leash walks or hikes it's a good idea to master a solid recall first. We work on our recalls and some basic obedience. Mostly we just let the dogs run and wrestle and explore and find sticks and dig. Ya know, be dogs! When I'm pretty sure they're all ready to crash is when we turn around home. The hike is all sand so it's low impact, even when they run around in circles like something has their tails!
Daily or bi-daily exercise will help keep your dog fit and healthy. It will stimulate his need to explore and to gather trophies (like rabbit poop! and sticks!) It will exhaust his excess energy and significantly reduce his boredom. Boredom for dogs means destruction for us. They don't just wake up and think, "Ew, green? That couch has got to go! Demo time!" Instead, they are bored and need an expression for their anxiety. And the couch was asking for it. Getting them out to explore new scents, new people, new dogs or new places allows them to stimulate their senses and their minds without any furniture-ial victims.
Before visiting areas frequented by other dogs be SURE your dog is fully vaccinated and never bring a sick dog somewhere that other people bring their dogs. If your male isn't castrated keep a close eye for other dog aggressive males or bitches in season. If your bitch is in season...seriously...don't go somewhere frequented by other dogs. Beware of dog parks which are great in theory but contain a high risk for virus and parasite exposure and you just don't know if you're going to run into that one guy who thinks he is God's gift to dog training but actually has some really aggressive dogs displaying behavior you don't want your Rottie to reciprocate!
I can't emphasize enough the benefit of having other dogs involved in your daily exercise. They run a lot faster than most of us do so if someone is going to be chasing someone...it may as well be two dogs. I can't run and drink coffee so...I'm out! If you have more than one dog, great! But also consider getting friends, family members, and neighbors involved. Scheduling regular outings together with your dogs will give them time to wear each other out as our pack wears one another out on our hikes. And in this circumstance you can control the dogs who interact with your dog! Choose balanced, social, well-behaved dogs who are not aggressive. Choose dogs you know are healthy and vaccinated.