Here's a picture of a tintero...
So now we know what a tintero is. The little bottle that holds your ink. Lucky for us the the ball point pen came along.
As usual, a literal translation won't work. There aren't to many things (cosas) you can put into a tintero. So let's just make our translation:
Tengo cosas en el tintero
I have some things on my mind
I happen to like this expression, don't ask me why. And I learned this from a Peruvian friend, so I really have no idea how universal this is, but don't let that stop you from using this phrase. If the people you're speaking to aren't familar with it, explain it. I bet in return they tell you their version of the expression and it's a win win situation for everybody.
That's it for today, ¡hasta pronto!