Is there a flying-rats / hidden packages equivalent in GTA V?

Looking at the checklist available on Social Club, there appear to be 2 collectible missions listed under Misc. There are also 2 others not listed specifically on there, which makes 4 total.

1. Collect 50 spaceship parts. This will also net you the From Beyond the Stars achievement/trophy. To start this quest, you need to be Franklin, and start the mission "Far Out". Some guides for this mission can be found here.
2. Collect 50 Letter scraps, which are available from the beginning of the game without starting any specific mission.
3. Collect 30 barrels of nuclear waste, which can be done after buying the Sonar Collections Dock for $250,000, and using the submarine.
4. Collect 30 submarine parts for Abigail, which is a side mission which starts on the Sonar Collections Dock (unsure if buying the property triggers this, or not).

Yes, there are hidden packages. I have found a few underwater in crashed planes and I'm sure there are a lot on land. There's apparently one near a military base with bodies around. Also, if you go into the water off the coast just north of the military base and find the plane wreck, the hidden package gives you 12,500 dollars. Switch to another character and back, and the package is there again.