1. Read something you don't typically read.
I get a lot of magazines that I don't remember ever subscribing to. Popular Science and Popular Mechanics are two examples. I typically don't read them as they arrive, but I don't throw them away, either. I often take them with me when I travel. On the plane, or at the airport, with nothing else to do, I read them. I also read the airline monthly magazine from the seat pocket in front of me. Not the usual stuff. You never know what will give you an idea.
2. Think about a problem you haven't resolved before you go to sleep.
You can force your subconscious to work on problems, especially if those are the last things you think about right before you go to sleep. When you wake up in the morning, you may have the solution. If not your problems, think about somebody else's problem. Think about a problem in the world. Then fall asleep. Your mind will continue thinking.
3. Take a different route to work.
When you drive to work, your brain is on "autopilot." If you change routes, you force the brain to think more -- and when you think more, you get more ideas. Simply thinking about how to take a different route to work will shut down your autopilot. You'll be amazed by how many different routes there are to work.
4. Talk to someone sitting next to you on the plane.
The probability that the seat next to you on the plane is empty is all but zero. Instead of keeping to yourself, try to develop a conversation with the person next to you. What do they do? What's interesting about them? By the time the flight is over, you will have learned something new.
5. Listen more than you talk.
When you speak, you don't hear anything new. Whenever you listen, there is a much higher probability that you will learn something new. When you speak with someone, especially someone new, the more you listen, the more you learn new things. Don't just interrogate them. Share something about yourself -- maybe even share a problem you're facing. You never know who has the solution.
Exercising is great for stimulating the brain -- especially in an unknown environment. Walking (or running) outdoors when you travel is much better than walking on a treadmill at the hotel's gym. There is much more scenery to stimulate your brain. When you are in the shower after that, new ideas will emerge.
7. Turn off the TV, don't check email.
Don't set the timer on your TV and fall asleep to the chatter. Don't turn it on as soon as you wake up, and don't immediately check your emails on your phone. Wait a few minutes. Stare at the ceiling. Let your brain make connections between old ideas and create new ones. When you watch TV or check email, the executive part of your brain takes over, preventing combinations of ideas.
If you don't have anything to do, don't force yourself to do something. Don't try to fill the void with your phone. Just daydream. What might seem a waste of time to your conscious mind is plenty of work for your subconscious. And that's when old ideas meet.
None of these requires significant effort or allocation of time. These are simple little things that will allow you to use time to get your next big business idea when you least expect it.