Summer time is a great time to start planning for the spring science fairs. By starting now you can gather data and information that can later be compared and analyzed, making your day and night, time, and season science project the best it can be. For instance, if doing a project about day and night you can start making notes on when the sun/moon rises and sets during the summer then compare it to fall, winter, and spring. Or you could make notes on the difference between the position of the sun/moon when it rises/set in summer vs. winter. These could also be used to start a time science project comparing the exact time of rise/set and how long the day/night is at different times of the year. They could also be used to make a science project about different seasons in the year, noting temperature difference or precipitation difference.
There are a couple great articles posted to educationbug.org this week that can give you even more ideas on how to expand on these ideas. There is a specific article relating to Precipitation Science Projects which gives step by step instructions on making a rain gauge. You could use this to compare rainfall in each season or day vs. night.