When you do the same type of activity for hours on end, you will find yourself less mentally sharp, more tired, and often more frustrated. A key to accomplishing many things is to switch to different types of activities throughout the day.
Some tasks require you to interact with people: phone calls, meetings, conversations with co-workers, engagement with clients. Other tasks need your technical mind: writing proposals, documentation, code, doing research. Yet others need you to be physically involved, such as working out, cooking, or cleaning. Some need you to be creative, some pragmatic, some playful, and so on.
To keep your day going smoothly and efficiently, try sequencing tasks in such a way that different types of activities follow each other. For instance, make a few phone calls in the morning, to get those out of the way. Follow that with a few stretches, to let your mind rest and your body engage. Then take on a technical task such as research, followed by a creative one, followed by a play session with your child. These can be done in whatever order, as long as each task allows you to engage a different part of the brain and body, and let those used in the previous task take a break.
You might also notice that after a physically-engaging task, such as a few stretches, a workout, or a brisk walk, your senses will awaken, which is likely to give you a fresh perspective on a mentally-intensive or a creative task. Additionally, while you are physically engaged, you might be working out a logical problem in the back of your mind, which will make it easier to come to a resolution once you are back to actively working on that problem.
The key is that you do not need to be doing nothing in order to rest. Relax different parts of your body and mind at a time while engaging other parts, and you will be more productive and feel more rested.