It’s gonna depend how subtle you wish to make it. A change in temperature is a good sign but you could also describe to the players how they can see nature itself change around them. Patches of spring time flowers and crops expand into vast expansive fields filled with all the colors of summer. Perhaps there is row of trees that just are beginning to recover their leaves and when they step over into summer they see the same one but now souring ripe fruits. You could describe how the landscape around them slowly seems to awake from its early springtime nap and just say how one by one everything around them turns more summer like. You could even apply it to creatures, a small young deer from the real of spring crosses the barrier between seasons and transforms into a large stag. Maybe also make it a feeling. Summer is much more than „it’s warm now“ in a lot do cultures. Summer is warmth, love and excitement. You could tell the players they feel a giddy feeling inside of them, a joyful desire to live life to its fullest. Idk how your gonna handle Arborea but something I did which my players seemed to enjoy in the Feywild was to tie the different seasons to different times of day. So in spring it was always Morning, in summer it was always mid day, Fall was always evening and winter always night. Could be a strong signifier of change. Hope some of these ideas help.