He gets rather desperate for playmates sometimes.
Here are five ideas that work for us a lot of the time.
1. Legos (actually, Duplo Legos) and other building toys like blocks or Magna Tiles. The Magna Tiles are a little pricey but they do make a great gift from the grandparents (thanks, Mom and Dad!) Although building toys are great on their own, add any other toy and you've got yourself a whole new bunch of awesome. Elly particularly enjoys legos and dinosaurs. I also highly recommend adding a hexbug to the mix.
2. Play games. Real ones. Maybe I'm crazy, but I taught my 4 year old to play Othello. And I totally wiped the court with him, but in a kind and gentle way. I like games for their ability to not make me pretend anything. There are the usual "age-appropriate" games like Candyland and Zingo. However, we prefer games with a wider age-range or that are actually for older kids. We like Jenga and dominoes (both of which double as building games) as well as Uno. We also like Spot-It and Set, both for much older kids. We play with "Ellis" rules for Set. He only needs to find one commonality between the three cards. He likes these "big-kid" games in particular. I think it makes him feel cool. The trick, though, is to make sure they don't feel frustrated.
3. Let them play with your camera. This is a bit scary. But it's not as scary as letting them play with, say, your iPhone. Ellis has discovered he can take pictures of himself. Now that was an easy 10 minutes of parenting.
4. Mud garden. I reserved a corner of my garden for Elly to just mess around in. He loves it. Combine the mud garden with the hose, spray bottles, sticks, clothes pins for maximum enjoyment. He had built a sort of dam using random wood and sticks. He loves to flood it and squirt it with water. It usually gives me enough time to get my gardening tasks done.
5. Yarn Play. Yarn and furniture can become a spider web, an obstacle course, a ropes course for "guys". We've also used it to make a "boat" of sorts. I cut lengths of yarn and we tie them to our dining room table, to chairs, to whatever. Ellis loves doing it and it works on hand-eye coordination and fine motor skills. He's learning to tie different kinds of knots and things like that.