One of the easiest and cheapest science activities you can set up for children of almost any age is a flower dissection. I especially like to use daffodils or sunflowers because they are readily available to me and they also have some clearly defined flower parts.
In this example (you can flick through the images below) my then 3 and 6yr olds took the flower apart, talked about what they found and created different pieces of art with them.
This was great for my younger child's fine motor skills (pinching and pulling off the petals etc, sometimes using tweezers) and for my older child
In another example, I made this very simple daffodil dissection board (just using cardboard).
I worked with my 4yr old and we looked up the names of the flower parts together first. Then she dissected the daffodil and placed the labels by each part.
This activity was free, fast to set up, she learned a great deal and had lots of fun!
At 18months old, my youngest also joined in by pulling a daffodil apart and I taught her the words 'petal' and 'stalk' as we chatted. My 7yr old took this activity a step further and used her microscope to examine the stigma further.
This is the microscope we got for her and we've been really impressed with it. It is reasonably priced and she's able to use it herself. It also allows you to place a phone on the eyepiece and see the magnified image on your phone which makes it easier for her to share what she's discovered with us all. Click the image if you're interested in taking a look (this is an affiliate link).