Here are some things my ds and I do together:
I made him a house out of cardboard, cut him a window and put a little shelf inside. He takes pics of cars (I cut out) and tapes them on the box. I give him sticker and he sticks them on the box. He puts his toy animals in the box also.
I made him a cardboard oven with a cupboard. While I cook, he takes the pots and pans and cooks, mixes, puts lids on, etc. He borrows my spoons and cups and feeds teddies.
My son loves his toy cars. He has a little madarine orange box full of them. He totes them everywhere. We make tunnels out of his blocks, we use the blocks to put in dump trucks, we even make garages out of shoe boxes.
Playdough and measuring cups are a lifesaver - pop in his highchair and away he works.
Crayons, felts and big paper. Watch the felts though.
Books- get a big bin and throw all the books in. Have it be accessible - rotate books every week or so from the library. Think of new themes too - eg. The Zoo - animal books - Things that go- car and truck books.
Tent and ball set. He loves hiding, and jumping and running into the balls.
Making towers out of anything! Once he took everything in his room and made a huge tower in the hallway. He called "mommy, mommy" and when I came he pointed to the tower full of stuff. He was so proud of himself. It had pillows, blankets, his potty chair, his puzzles, his clothes, his diapers - you name it!
On you mark, get set, go! On go- I raise my arm and let the little guy race to the other end of the hall. He loves it and races all the way back to do it again!
Going for a walk always breaks the day. Even if it's only for 20 min. It's good to get outside.
Playing with velcro curlers - As I do my makeup, my little guy likes to take out my velcro rollers and play with them. Usually, he takes the small one out of the next one then out of the medium one to the large one. There are 6 big ones all filled like this. He gets them all out and is quite proud of them all on the carpet. He also likes how they stick together.
If you are like me and leave your laundry till last minute when you have 5 baskets full- this makes for fun time folding. Little son likes it when I dump the big loads of dry clothes on the carpet. I fold and he helps but when he gets bored, he likes to drive his toy cars through the bumps the clothes make. After everything is folded, he helps put things away.
Dancing with Raffi - my ds is hooked on Raffi. He dances and spins and runs and hops- it's a great time.
Playing doctor. Just now, my son will lie on the carpet and say Help. I pretend to call 911 and take him to his room. I tell him not to move and go get Dr. Teddy. Dr. Teddy takes my son's temperature, heartbeat, gives him pretend medicine and water. Then a hug and lots of rest. Then we play it all again. The same procedure but he sure loves it.
A play car mat is wonderful. I used to make them on paper which was fine but then I found one that is a carpet at Home Depot. What a great toy! My son loves driving his cars along the roads and stops at the bus, school, police station, etc.
Blowing bubbles/ looking at pictures of mom, dad, grandma, etc.
I filled an old purse with old cars - like old credit cards, movie rental cards, used gift cards and canadian tire money. My little guy loves going throught the purse and pulling out all the cards from the wallet. Eventually, I will fill it with toy keys, and photos, and whatever else I can think of.
Well, hope these help. Thanks for everyone else's suggestions.