Never underestimate just how much fun balloons can be!!  Great to play with in the house (with careful supervision if you have young ones).  Balloons don't hit you in the face and give you a black eye or hit a shelf and knock off knick-knacks (paddy-whack, give a dog a bone.  Sorry, couldn't resist).  

Bubble Baths  

Use plain old baby shampoo if you don't have bubbles.  And you can use the colored bath soap. Never resist the chance to stimulate the imagination. Pretend to sail a big boat, look for fish. 

Tent/Fort  

The good old stand-by from many previous generations.  Always fun! Bring out the stuffed animals to add to the "camping" experience! Pack a "picnic" lunch and make sure to include marshmallows. Tell campfire stories with sound effects of clapping, rubbing hands together and so forth.

Make muffins!  

Buy the small bagged mixes.  Toddlers can help combine ingredients.  Maybe then toss a blanket on the floor and have a "muffin picnic". Older children can "experiment" with the mixes. How does it taste when you add some chopped nuts or mashed banana?

Dishes  

Young-ish kids love to play in water so get a safe and sturdy step stool and let them help wash non-breakable dishes!  And then pull out the mop and wash your floor afterwards!  ;) 

Gardening

Spring has sprung finally!! Yes, you can garden with kids. You can use a variety of things to start seeds in - milk cartons, Jiffy seed starters, egg cartons... Poke several small holes for drainage in the bottom of the container and add potting soil. The child plants the seed in the soil and waters. I do recommend the small watering cans with sprinkle spouts on them for children. You don't run the risk of them washing the seeds out. Place in a window or another spot that gets good sun and sit back & watch! The important note section: Be careful what types of seeds you buy! For kids, do not get seeds that take very special care (i.e. "needs light to germinate, do not press into soil"). Those will only frustrate younger kids. An excellent seed to try is Nasturtium. The whole plant is edible, striking flowers, requires no special care and seeds are large and easy for little fingers to handle.

Make a marching band!

Take old plastic baby wipe containers or Rubbermaid bowls for drums. Get a resealable can with a plastic lid and place in it dried beans, pennies, etc. for shakers - you can also wrap it with colored paper to decorate making sure that you tape or glue gun the lid securely shut. Paper towel tubes for "wind instruments". Fold paper hats and put on bright clothes. Then you are ready to march in your own parade. Great for those short walks to the park, the kids stay entertained on the way!!