You could try any of the following ideas or activities below:
Make a picnic and enjoy it at a local park.
Build a den in your garden.
Visit a pick your own site and pick strawberries or other fruits and vegetables.
Draw or paint a picture of things you find outside e.g. flowers, birds, views.
Take photos of different views and landscapes. You could sketch/paint them afterwards.
Create a treasure hunt/scavenger hunt.
Read a book outside.
Make a simple bird feeder.
Create an obstacle course in the garden.
Camp out in your garden, if you have a tent.