Make Blocks
You can start making a stash of these and once you get plenty in your hoard, your toddler can use them much like a set of blocks. You can even reinforce them by stuffing them with newspaper or other paper that was destined for the recycling bin. Make them look great and add even more durability by covering with colored or patterned contact paper. You can also just let your toddler paint or color the tubes with crayons or markers.
Make Sound
Toddlers also just love to hear the sounds of their own voices through these tubes, so take them outside and encourage some yelling, talking and whispering. Experiment with the different sizes and point out how the size changes your child's voice.
Pretend and Encourage Large Motor Skills
Stimulate imaginative play and large motor skill development by suggesting other uses for the tubes (though your toddler will surely come up with his own ideas, too). You can suggest that it's a walking stick, a sword, a guitar or a microphone, a telescope, a bat or a golf club (maybe even hit some pine cones or foam balls on the ground). See how long your toddler can balance one on his head while standing still and then walking. Be sure to join in the fun so you can both laugh when they tumble to the ground.
Improve Fine Motor Control
Use cardboard tubes to help develop fine motor skill by threading a piece of string through it or filling one that's standing upright with cereal at snack time. Try to get your toddler to spin one around while holding it on a pencil.
Make Some Art or a Gift
There are a number of crafts you can make from cardboard tubes, including gifts that can be given to friends, teachers and family members. Two of my favorites are elephant trunks and the musical Rice-a-Racas.