First of all welcome to our game!
Second it is a great idea to remove the container slightly to behind a bush when opening it or into your vehicle (if its a park n' grab) so that you can be discreet if possible. Some cachers do fill up a container a bit much with certain items, usually it isn't the original hider that has done so. The polite thing to do is to remove something large and place it in a more appropriate cache if possible and swap something smaller into it. Or just leave it if you're not a swapper. It is most important that the logbook remain in a ziplock. The rest of the stuff just gets shaken back into place if we decide not to remove anything.
Third calling cards are something some of us leave when we take nothing/leave nothing or we carry them along to place in caches where the logbooks are full. They are free for the taking as far as we are concerned. We collect calling cards from other cachers just for our own interest sake! We like to see the creativity others come up with.
If there are many muggles in the area it is better to try to wait for a better time to try for the cache...Or pretend you've dropped your own package..or pretend to be answering a phone call on your GPS. There are many ways to distract muggles. Most important is that they don't see you remove or replace a cache even if they see you playing with the stuff. Think of how it looks to others. Few people actually see you unless you seem covert.

Good luck - I'm sure other cachers will have more tips for you. Just enjoy!

Geez.  I was trying to make you all laugh!  Nobody else has a funny story to add!??