One of my first questions on mamapedia was similar. My niece had just come to live with us, so we were in the transition of becoming a two toddler home and I was going from being the bread winner to the SAHP again. Uvy, tricky times! What I've found is that ANYTHING can be a fun toddler activity. Let's unload the dish washer! Let's pick up toys! Let's put all the white dirty clothes into a basket! Let's dance, dance, dance! Who wants to put on funny hats and make faces!
Also, down time has been a life saver for me and them. After some time and a lot of practice, they've gotten to the point where they can (or I can let them) entertain themselves for twenty minutes to two hours. They will play with dolls, make "tea", pretend one is Mommy and the other baby, "fly" around the house, etc. Internal play is very good for their development and *absolutely essential* to my well being ;) We'll rotate between a more structured activity (going for a walk, eating lunch, doing an educational game) to them just playing by themselves. It's a balance that is really important, in my home.
Forts are fun, legos (just graduated to the middle size in our house!) are great and they got a doll house that keeps them entertained also. Another favorite is to put them in the kitchen (tiled) or the shower and let them play with water. At first, they were just dumping the water from tin to tin, but now they "go fishing".
*Washable* markers (I learned my lesson with the wrong kind of pen/marker), paper from the hardware store (cheap and comes in roles), paints, crayons, pencils and stickers are messy, but very fun. I draw pictures at the same time. Play doh is a mess, but also a favorite. We bring in forks, butter knifes and other common household objects to "sculpt" what we want. All I can say, is I'm glad my house isn't carpeted. My present to myself, once the kids are out of these years, is to buy myself a new, not nasty, rug.
We have to go outside every day (sick days are an exception) or my house becomes a mad house. Walks or the park...either works. We also recently got a membership to the Hands on Children's Museum. Totally worth it.
We get together with my Mom friends who have children around the same ages (or not, for that matter) for social stimulation. I have a good time gabbing with my girlfriend while our hoard hurricanes through the house. God bless child proofing.
We also do spend the nights with the same group of Mamas and kids. My SO works funky hours so it's wonderful to make dinner with another adult, feed the kids, get worn out, put the kids to bed and then stay up drinking wine and talking while the kids sleep. It's been a life saver.
Like I said, our transition was a little wonky at first. I had many a "oh my goodness, this is harder than I thought it would be"/I'm ripping out my hair moments. But now, things are in a groove and it's actually really fun. Like anything, it just took time and practice. Good luck!