I had a fairly extensive coin collection that I inherited from my father (who had been in World War 2). But (several years ago) I let my daughter take the album to elementary school and it disappeared. (sigh).
But yes, when I'm visiting a foreign country I like to bring back commemorative stamps... it's an inexpensive, beautiful, and lightweight souvenir!
Some of the stamps have a space or astronomy theme... I have an especially beautiful space set from Australia and some nice space ones from Madagascar.
Coins and paper money are also easy to collect but I don't recall any with the sun on them.