On the right - the name and address of the person you are sending it to.
The standard would be:
"Having a wonderful time. Wish you were here!"
You can jazz it up a bit with details of your days - Temps in the 80's. Went swimming and built sand castles yesterday. Going to EPCOT tomorrow. Saw the world's largest ball of string! Had dinner at Rainforest Café.
Short note about how the trip is going, how you like the particular environment. A note about the weather and how you miss your friends- parents whatever.
A list of every item of underwear you own.
A poem about homeless bald people.
A shopping list.
How the hell do you expect anyone on here to advise you what to write? We don't know you, we don't know where you are, we don't know who you are sending the card to, or anything else about you.
Surely you can work out what to write yourself?
Address the card and wet it with a signature scent.
say I LOVE YOU
