If you live in an area where oak trees drop their acorns in the fall, you can collect them and start your own oak trees. This is an interesting project for a young child to do with a parent or grandparent. Together they can watch the tree grow each year until it is large enough to play in or swing from. It won't cost anything and it is a good lesson about investing what you have now for the future.
Collect the acorns in August and September as soon as they fall. The longer you leave them, the more chance they have of being infested with worms or hungry squirrels eating them. Remove their caps and wipe them clean.
Dampen a couple paper towels or napkins and wrap up several acorns. Place them in a plastic bag, close it up and place it in the back of the vegetable drawer of your refrigerator. They need to stay here for three months--simulating the resting period during winter.
Fill a plant pot with regular garden soil, potting soil, or whatever you have around. The acorn will only be in the pot temporarily so the soil type is not very critical at this stage. Make sure there are holes in the bottom of the container if you are using a coffee can or other such type of container.
Remove the acorns from the plastic bag and the damp paper towels. Be careful in case some of them might have already started sprouting. Place them in the container, burying them about 2 inches deep, firming the soil back over them.
Set your planted acorns in a moderately warm spot (50 to 60 degrees F) and give them enough water so that it drains from the bottom. A tray will keep the area clean from drainage water. Water them every few days when the soil starts to feel dry.
Take the seedlings outside to a sheltered and partly sunny spot when there is no chance of frost, especially in the evenings. They should be transplanted into the ground after a few months of being outside or at least into a three- to five-gallon container to protect their roots from freezing in the winter.