I've gotten 41 out of 42 Hobbies and Pastimes on GTA 5, and can't find out what the missing one is. I have tried Stock Car Racing, but can't find it at all on the map. I also can’t find anything to do with the Wildlife Photography Challenge, so I'm lost.
can be found near the Yellow Jack Inn in the Grand Senora Desert and is marked on the map with a white race icon.
An NPC will be standing next to a racecar that will become the prize for that race. The player can join the race using limited car classes (e.g. supercars are not permitted).
It's close to the north-east point of the Sandy Shores Airfield.
This counts as part of the Hobbies and Pastimes challenge.
is an exclusive side mission for Franklin in the enhanced version of Grand Theft Auto V for players returning from the PS3 or Xbox 360 edition.
So this challenge might not be accessible to you.
In case you do have the enhanced version:
After completing the first Strangers and Freaks mission for Beverly, you will receive a text message saying he's signed you up for the Wildlife Photography Challenge. At the same time, an email from the LS Tourist Board will arrive, giving you details. The animals must be photographed up close using Snapmatic on the phone, and must be centered in the frame, with no obstructions. Once photographed correctly, a prompt will appear to email the photo to the LS Tourism Board, whose contact info appears in Franklin's phone.
As for your missing hobby/pastime, besides the already mentioned Stock Car Races, they include Arms Trafficking Air, Arms Trafficking Ground, Bail Bond, Darts, Flight School, Golf, Hao Street Races, Hunting, Off Road Races, Parachute Jumps, Sea Races, Strip Club, Stunt Plane Time Trials, Tennis, Triathlon, and Yoga - all in all 59 options.
This answer on the Arqade already lists them.
Here's the overview on IGN.com, with some additional links.
Easiest, though, would be to go to Rockstar's website, and look at the overview of your playthrough, which you can do right here (you might have to manually sign in).