I get this sort of thing all the time. I've had everything from photos of random grandchildren through reciepts for firearms and royalty payments for an episode of The IT Crowd, to gynaecologist appointments. I once got invited to the civil partnership ceremony of a lecturer at $british_university that I had to think really hard to work out whether they knew me or not.
No invitations to come and play with anyone's submarine, thankfully. Though I did get one yesterday from someone asking where the umbrella was. I assume that was a coded message from a spy.
Most of them are sign-up confirmations from online services. Some of the better ones have a "I am not the intended recipient" button, or at least unsubscribe facilities. Sometimes I have to log in and change the email address on the account to support@service.com.
But the real ones from friends and acquaintances of people who don't know their own email address are harder. There's almost never a clue as to who the intended recipient actually is. The law of averages suggests that the friend of a luser is also a luser, and if you reply to them they'll just add your real address to their contacts list, which will inevitably get added to one of those CC-to-everyone-in-my-address-book emails and sent to *other* friends of the original luser, and then you'll never escape.
If it's particularly important looking, or from a professional who's likely to have out-of-band contact details on file, I will reply and point out the error.