Put a “price” on some birds you will likely see on your walk (let the kids do it), then on the walk whoever spots one of these birds gets the points rewarded. If somebody sees one that it not on the list, they must find out the name (can ask mum or look it up in a bird book…) and then the kid who saw it first can set the “price” of this bird.
You can include other animals: snails, ladybugs, caterpillars. Be more detailed with older kids (not just caterpillar, but different number of points for different caterpillars).
Only the kid who saw the bird first gets the points, and the same bird cannot be "discovered" again when it flew on the next tree...
Instead of making it a competition among the kids, it could also be a family effort (cumulate the points) and then depending on how many points you achieved, donate to a nature or wildlife charity the end of each season.
You can put lentils in a char to demonstrate the increasing levels of birds spotted.
Find an old tree with many woodpecker holes, broken off branches, gaps between the roots, etc. It ought to be an interesting old tree.
Then let the kids draw it with all the many holes and hiding places and write or paint who might live in which hole. This does not have to be a realistic picture, it can look a bit like an architectural plan of a house, or a puppet house.
Tree hole competition
Go on a park or forest walk, and count as many tree holes as you can find. Older kids could track the trees on a map: where were they, how many holes did you see in each tree? You can also send half of the family (with dad) around the left half of the field, and the other half (with mum) around the right half… walking along the tree line and counting tree holes (or birds, or whatever interesting things might be there).
Tree hole collage
Take close-up pictures of tree holes, which the kids discovered, print them at home, and make a wall collage with all the tree hole pictures on a dedicated wall at home. The kids then can cut-out birds, beetles, squirrels, and other critters who may live in these holes and attach them with blue tag. They will have to go out and "collect" more tree hole pictures to grow the collage and allow more cut-out paper critters to live on your wall.
Finding Bug Hotel materials
Explain to the kids what a bug hotel is and show them some pictures. Then go out to collect things that you could use in the bug hotel (bark, dry grass, broken branches, etc.).
Tree bud detective walk
Collect some pictures of tree buds. Make a check-card with some of the common ones for each child (in each row: tree bud picture on the left, write-in area to the right, stamp area). If they see a certain tree bud on a walk, and recognise it from the picture on their card, they can write down where they saw it, and get a stamp at the side of the picture. When the card is full, they get a treat.
For larger kids:
Use your smart phone to record and look up bird song (you need to use the video function). You then can use rspb.org.uk/birds-and-wildlife/wildlife-guides/bird-a-z, which have sound bites).