Masking-tape sculpture

Stack 3–6 cardboard tubes (paper towel, toilet paper) and a roll of tape on the floor. Pre-tear a few pieces of tape and stick them lightly to the edge of a table where the child can grab them.

Put two tubes end-to-end. Tape them together. Look at it.

Towers. Bridges. "Telescopes." Then the question "what if I tape this one here?" Genuine engineering happens after about five minutes. A 2-year-old will mostly just rip tape off and stick it to themselves — also fine.

Add a paper plate as a "roof." Tape the sculpture to the floor for "permanent" mode. Replace tape with stickers — different connection, same schema.