I love the idea of Postcrossing, it's a good way to travel from the comfort of your armchair
As for that question-what cards would I like? It's the message on the back that is more important then what is on the front.
If you're stuck on what to write-what you did during the day, what the last book you read was. Or, your favourite recipe. I'd also really like the date on your card, and your weather too.