We really like this activity because it's another one that's really flexible and can be modified to suit everyone (unless they're really Not In The Mood). It can make anything from giant splodges of colour to really delicate leaf imprints, depending upon what you use, how you approach it and how patient you are.
The colours won't last forever as they're stains not dyes so will tend to fade in sunlight or with washing- they make great pictures or cards which will last for months rather than years as a rule- don't expect them to be heirlooms.
You will need:
Paper or plain, pale coloured fabric (yes, this is another use for scrap fabric!)
A hammer or rolling pin
A surface you're happy to hammer onto- preferably not too textured. Putting an old thin cardboard box like a cereal packet onto concrete is a good idea, or rest on a piece of wood.
This is quite a loud activity- people with sensitivities may want ear defenders, and in any case it's more pleasant outside!
1. Pick some leaves or flowers. Grass stems and dry-feeling leaves and flowers tend to make crisp images; very fleshy leaves or flowers tend to make a bit of a squishy blob.
2. Arrange them on half of your piece of cloth or paper
3. Fold the cloth or paper over to cover the leaves or flowers
4. Hammer carefully all over the leaves or flowers until you can see the outlines through your material
5. Open the paper or cloth back up, and gently pick off the leaves. You may find this step easier if you wait until the pieces have dried off a bit (and yes, this has magically turned into a completely different leaf picture!)
6. Repeat all the steps if you want to add more pieces.
7. Once you're happy with your picture, leave it to dry. You could then cut out pieces to use on birthday or get well soon cards, use larger pieces of fabric to wrap presents, or hang the pictures on your wall... or whatever else you like!
Troubleshooting:
People who haven't had much practice with hammers or who have difficulty with coordination tend to hit wonky, using the corner rather than the flat face of the hammer. This will make small crescent shapes instead of round shapes as they hit and makes crushing the leaves difficult. You can encourage practicing hitting straight to make circles not crescents, or switch to rolling with a rolling pin if hammering gets too frustrating. A rolling pin is less effective with hard, delicate or dryish leaves than a hammer, but works well with wetter, softer leaves and flowers.
Paper will tend to tear if hit with a corner of a hammer repeatedly. If this is an issue, either switch to a rolling pin or use fabric, which is a lot more forgiving of wonky hitting.
Different weights and absorbencies of paper and fabric work best with different kinds of leaves and flowers. Printer paper isn't very absorbent, so fleshy flowers can make quite a mess (depending on your definition of "mess"- one of us likes the spatter patterns you get). Thicker canvas will make a tidier imprint of these kinds of flower- but is harder to get a good image of drier herbs or intricate flowers onto. I like using shirt-weight fabrics and old sheeting because you can make prints of tiny delicate flowers and get whole leaf vein systems to show. The kids prefer paper "because it sticks better on my wall".