There really is no indication at first that anything is happening - you can hear the engines increase a little as the plane starts to climb but since there are no windows you have to real clue that anything is happening. Slowly you start to feel the increase in gravity as pressure across your whole body and if you try to lift your arms they really feel heavy. Its not uncomfortable but very odd but then quite quickly there is no pressure and you see people pushing off and WOW you feel very light. Martian and lunar gravity is strange enough but the first zero g is even weirder - you are suddenly not touching the floor and the slightest movement makes you go feet up or collide with your team mates. You can see why they say no jumping - you find yourself on the roof without even trying.
After 30-40 seconds someone yells 'feet down' and you have to make sure your feet are closest to the floor. Gravity comes back quite quickly and you will hit the floor hard. This is why its padded. They have us do some fun things like chase M&Ms and play with water. Of course when gravity comes back you are showered with the spare M&M and lose water but that's all part of the fun.