Have them play/practice with pattern blocks and then find ways to make a trapezoid, rhombus, hexagon with smaller shapes.
Make a shape outline and have students fill it with pattern blocks or let them make a shape out of pattern blocks...for each, they must total the number of each pattern block used as well as the total number used.
Hope this helps.
Also,make sure they understand the orientation: is a triangle still a triangle even if it looks upside down?
For 3D shapes, spend some time exploring. Students record the number of faces, edges, and corners. Point out non-defining attributes (it is a cube because it's blue, or small? No! It's a cube because it has 8 corners, etc...). Then build 3d shapes with toothpicks (edges) and gumdrops (corners). You'll have to show how the faces are invisible here. You can also have kids go on a scavenger hunt in the classroom to find 3D shapes or 3D shapes.
Spend time building new 2D shapes with other 2D shapes, then recording (I built a trapezoid using 3 triangles, I built a pentagon using 1 trapezoid and 1 triangle, etc.).
We always refer to spheres, cubes, rectangular prisms, pyramids and cylinders as solids. We do activities with counting corners. e.g. If there are 3 cubes and 2 spheres how many corners? Answer 8 + 8 +8 +0 +0=24