I am frequently poo-pooing TI's offerings on their website around here, so I thought I'd balance the karma by advertising a pretty decent activity we did today: Midpoint Quadrilaterals.
It has the right amount of intellectual rigor and things for the kids to actually do, and the documents that suggest a progression of inquiry did not require much modification. There are four parts, and for each part I made sure they knew what they were expected to do and look for and write down, gave them time to work, then regrouped and had them share their conclusions and reasoning.
I also inserted a task before proving that the inscribed quadrilateral was a parallelogram (after Page 1.3) to remind them of triangle midsegments which looked like this:
...which if this interests you enough to check it out, I'm sure you can figure out why.