I've been using a Varidesk review unit for the past four weeks, and I love it. It's a platform that sits on your existing desk, and allows you to easily raise and lower your computer and keyboard, so you can sit and stand throughout the day.
I typically start my workday standing up, and I'll alternate between standing and sitting an hour at a time. I've been using the Pro Plus model ($(removed)), which is the largest one they make, and I'm glad I didn't get the Single, because I like to keep notebooks, books, iPads, phones, etc. within easy reach. The Pro Plus has enough real real estate to hold that extra stuff on it. The Pro Plus has two platforms – one for the computer/monitor, and another for the keyboard. The Pro model ($(removed)) has only one platform for the keyboard and the monitor, which means you'll have to tilt your head down to look at the display (or elevate your monitor with a stand).
As you can see in the above video, it's easy to raise and lower the platform. You grip the levers on either side and guide the desktop to one of several different locking positions. The hinged braces have springs to assist you, so it's pretty effortless. The fully extended position would probably work with a treadmill, which I want to get soon.
One thing that is a must with a standing desk is an anti-fatigue floor mat. I've been using Varidesk's branded mat ($(removed)) and am pretty happy with it. But after an hour, my heels hurt and I'm ready to plop in a chair.
I've experimented with homemade standing desk contraptions for years, but I always abandoned them after a few months because I didn't like not being able to sit at my desk. The Varidesk solved this problem. I intend to use it for the rest of my life.