My first experiment involves measuring the density of water. Each group of two kids is assigned a specific volume of water from 10 to 100 mLs on the tens. They simply measure the mass of an empty graduated cylinder and then add the water and find the mass again. Once they have their data they go around the room and find another group that has one of the volumes that they need and get the data from them and record their names. Once complete they generate a graph of the data and answer a few simple questions. The whole procedure can be completed in about 20 minutes.
Each spring my Local Section of The American Chemical Society (ACS) hosts a rigorous two part exam as part of the selection process for the The International Chemistry Olympiad (IChO). The lab practicals on the exams are presented as problems. No procedure is given. Students must use their chemistry knowledge and lab experience to devise a plan and solve the problem. The best part is that the released exams come with lists of materials and equipment, helpful hints to the proctors, and solutions! This is a great source for Inquiry-Based Lab Practicals.