One of the hardest geometry concepts for 2nd and 3rd grade is solid shape characteristics. I remember my students needing constant repetition and practice to understand how many faces, edges and vertices each solid figure contained. I recently found this AMAZING website by Houghton and Mifflin that is great practice for this skill:
There is even a section that shows the net of each figure that unfolds in real time!
Be sure to check out the other etools this website has to offer. Click the image below: