Go to the library.
Go to the park.
Go to the play place at the mall.
Sign up for a "mommy and me" gymnastics or "music and movement" class.
Sign up for a Moms Club.
Trade kids with another mom in your neighborhood - have her child come over one day a week for a couple of hours, and send your kid to her a second day for a couple of hours.
Visit relatives.
Visit a museum or zoo.
Go shopping.
Have a friend who also has children over for coffee.
Play in the yard.
Go for walks.
Make cookies.
Paint.
Play with clay/play-doh.
READ.
Turn on some music and dance in the living room.
Make cookies.
Find an "indoor playground" - here it's called Monkey Joe's and Chuck E. Cheese.
Find a church that has a "Mommy's Time Out" program and drop him off so he can play with the other kids, go get a pedicure.
Join a gym that has free child care - drop him off so he can play with the other kids and go work out.
Go to the pet store and look at the critters.
Go to the animal shelter and look at the critters (beware of the dogs - our shelter is full of pit bulls all the time, but the cats and bunnies are nice).
Go to outdoor concerts/festivals - our town has "brown bag lunch" concerts during the week right around lunch time. The people who work downtown can bring a lunch and listen to music during their break, but there's no reason why kids couldn't come.
Babysit someone else's kids - you'll be amazed at how well someone else's kids entertain YOUR kid!
Volunteer to help out in your older kid's classroom/school - provided that you can bring the little one along.
When I had "two under two" I tried to do one thing each day - some days it was just grocery shopping, but it got us out of the house and my kids were MUCH better behaved and less bored when we weren't just sitting around the house looking at each other all day. Now they're 4 and 2.5, and they're in preschool and activities and the weeks just fly by.