I would like to complement the previous answer.
If you and your friend are close, your can also end your informal letter with good wishes like:
"Nos vemos pronto!" (See you soon!)
"Saludos a tu familia" (Greetings to your family)
"Que tengas un buen viaje" (Enjoy your trip)
"Un fuerte abrazo" (Hugs - strong hug)
And then your name, to complete the structure of the letter:
Nos vemos pronto!