Melissa,
We make forts out of kitchen chairs and blankets.
Something else I do sometimes is go to the grocery stores in the mornings when they are stocking the shelves and ask for some empty boxes, if you close them up they make great big huge blocks for stacking- kids can build walls or houses (with a blanket for a roof) if you don't mind the mess, and every week you can add a few more boxes. Cereal boxes could work good for this too.
Or get bigger boxes and let them play directly in the box itself.
Let them take a bath and put food coloring directly in the bath water. What I do is put about 5-10 drops of coloring into an empty shampoo bottle and then fill the rest of the way up with water. Let baby dump it out if he wants, once its diluted a little it will not stain them or the tub. Sometimes I do 2 different colors in 2 different bottles and dump each in opposite ends of the tub, then watch the colors mix in the middle right around the baby, very exciting!
We also love YOUTUBE we look up kids songs like chicka chicka boom boom, or schoolhouse rock songs, there was an old lady who swallowed a spider, or even just upbeat songs that get you moving, sometimes we just sit and watch them or sometimes we jump around and dance to them, depends on our mood that day. Just about anything you can think of is on there.
Sometimes instead of a regular lunch we will put a blanket on the living room floor and have a picnic. finger foods or snacks. It can be a nice change of pace.
I also have a small little tykes slide that I brought in and put in my basement, helps burn off a little extra energy sometimes.
Hope this gave you a few new ideas.