A traditional yogic celebration can include, of course, sun salutations or Surya Namaskara. To hold your own Summer Solstice celebration consider doing 27, 54 or 108 Surya Namaskaras. This group of poses can be considered basic. But, string a number of them together, really focus on your breath and you may enjoy the challenge. The first time I did a set of 108, I was surprised at how much focus it took, how physically demanding it was, but also how lost I got in the flow of the movement and breath.
For added effect practice your Surya Namaskaras outside or in silence. After this incredibly rhythmic practice, take a few minutes to meditate on the gift of the long day, the sun or your yoga practice. For this soltice, I'll be using St. Francis of Assisi's Canticle of Brother Sun for my inspiration. I've included it below, maybe it can be your inspiration as well.
The Canticle of Brother Sun
-St. Francis of Assisi
Most High, all powerful, good Lord,
Yours are the praises, the glory, the honor,
and all blessing.
To You alone, Most High, do they belong,
and no man is worthy to mention Your name.
Be praised, my Lord, through all your creatures,
especially through my lord Brother Sun,
who brings the day; and you give light through him.
And he is beautiful and radiant in all his splendor!
Of you, Most High, he bears the likeness.
Praise be You, my Lord, through Sister Moon
and the stars, in heaven you formed them
clear and precious and beautiful.
Praised be You, my Lord, through Brother Wind,
and through the air, cloudy and serene,
and every kind of weather through which
You give sustenance to Your creatures.
Praised be You, my Lord, through Sister Water,
which is very useful and humble and precious and chaste.
Praised be You, my Lord, through Brother Fire,
through whom you light the night and he is beautiful
and playful and robust and strong.
Praised be You, my Lord, through Sister Mother Earth,
who sustains us and governs us and who produces
varied fruits with colored flowers and herbs.
Praised be You, my Lord,
through those who give pardon for Your love,
and bear infirmity and tribulation.
Blessed are those who endure in peace
for by You, Most High, they shall be crowned.
Praised be You, my Lord,
through our Sister Bodily Death,
from whom no living man can escape.
Woe to those who die in mortal sin.
Blessed are those whom death will
find in Your most holy willl,
for the second death shall do them no harm.
Praise and bless my Lord,
and give Him thanks
and serve Him with great humility.