Use this phrase to explain what you're doing, and make it sound like a sudden decision rather than something which was planned out in advance.

I just thought I'd call and see if you wanted to have lunch some time this week.

You could also say it this way:

I wanted to call and ask if you wanted to have lunch some time this week.

But "I wanted to ___" doesn't tell how long you wanted to do something. It could mean any of these:

• You've wanted to do it for a long time.
• You just decided that you want to do it.
• You wanted to do it in the past, but now you don't.

"I just thought I'd ___" expresses the idea of deciding quickly and without thinking too carefully about something. Here are some other examples:

No, there's no special occasion; I just thought I'd clean up a little bit.

Hey. Are you OK? I just thought I'd see if you needed any help.