I’ve made jigsaw puzzles with the grandkids help in the past using a thick cardboard & pictures of their choice glued on before cutting them out.

I use milk bottle tops to play noughts and crosses by drawing the grid and having 2 different coloured tops one for each player.

Toilet rolls are great for making animals, rockets, castles and windmills. Egg cartons make great chickens, goggles, hanging mobiles and caterpillars. I’m sure your 3.5 y/o could easily entertain themselves with all the choice of re-usable material you have.

Old shoe boxes are always good.
You can create little scenes like under the sea using pasta shells etc.
You can also create houses and cars out of them.

Honestly I’d just give the bits and pieces to your 3.5yr old and let them do whatever they want themself! Kids come up with the most creative ideas on their own!

Sometimes its best to just let the children create their own ideas with a little help from you. If you set up a box construction area with a variety of materials with some glue and sticky/masking tape the children are then free to use their imagination rather than trying to replicate something you have shown them.

If you want some long term projects, consider using some of the materials to start a garden. Toilet paper rolls make excellent seedling starter holders, as do egg cartons. They can then be sown directly into the ground! The milk bottles are great watering cans, but I’ve also cut them and grown sunflowers in them.

Pinterest is my best friend for these.ideas.

Today we’re having a pirate day with my 3.5 year old.
Made a pirate ship out of a big cardboard box
Made a spyglass – either stick 2 toilet paper rolls together or use 1 paper towel roll. Get some cellophane or clear plastic wrapping from food packaging and draw a small pirate ship silhouette on it with texts (I found a picture on my phone and traced it). Stick the cellophane over the end of the tube so the pirate ship sits in the middle, then decorate the outside of the spyglass with paint, textas, tissue paper, or whatever else you have. When they look through the tube they’ll see the ship which is fun for that age.
Make a treasure chest – use an old egg carton and paint it brown or stick brown paper or tissue paper on it. Add decorations yo it with whatever you have handy and fill with treasure (I used it old lids and stuck yellow paper and drew skulls on the)
Treasure map – hot glue wet wipe tops to a firm piece of cardboard. Number them on top in the order of the treasure hunt and draw picture of spots in your house on pieces of paper (or write clued for older reading kids) that you can then blue tak underneath (that way you can change the order later if you want to). Starting at 1, get your child to flip open the wipes lid and see what the clue is and get them to go to that spot in the house. When they get there have an activity set up for them to do (eg walk the plank, do 10 “starfish jumps” etc) When they finish the activity they can open the next number and go to the next spot. At the end, have the treasure chest you made waiting for them.