For me the Genesis song that best casts a summer-like feeling is
'In Too Deep'.
The following one is 'On the Shoreline' (curiously 'No Son of Mine' has a winter mood).
Then next one, 'Throwing It All Away' and then, without any particular order, 'Invisible Touch', 'Paperlate', 'You Might Recall' and 'Way Of The World'.
And I also think 'Domino' has a Midsummer evocation, I always play it every 23rd June.
Other songs with summery vibes are:
'Anything She Does', 'Follow You, Follow Me', 'Home By The Sea', 'I Can't Dance', 'Illegal Alien', 'Jesus He Knows Me', 'Land of Confusion', 'Mama', 'Squonk', 'Taking It All Too Hard' and 'Vancouver'.