Quad bikes were originally designed for farmers and are in essence, rugged four-wheeled motorbikes. There are several formats to these activities so think over before choosing the right one for you.
Quad trekking is superb provided the centre has the extreme terrain required to make it interesting and enough quads to cater for your group. You want to test the bikes on tough terrain to get the most from the experience and the centres do vary in quality.
Quad bike racing is challenging and competitive.
Generally, you'll be racing around a purpose-built off road course in a grand prix format with heats and a final. Again you'll want to ensure there are enough bikes to minimize waiting around time.