If you haven't noticed, I'm on a roll lately with activities using materials I already have around the house. This "Build-a-Shape" activity using birthday candles was a lot of fun. I only had enough candles for B to build one shape at a time, but it still worked out well.
Here is the Candle Build-a-Shape mat to download or print.
I've also seen something similar using popsicle sticks or craft sticks. If you have those on hand, build the shapes on a piece of paper and then trace them to create the mat. Give your child the mat and sticks to build the shapes.