[Samsung Galaxy Nexus] ashclepdia:
Just take a looksie around this forum section. TONS of threads with full detailed information on how to go about it.

I'm not sure about the activation on another carrier part as I've never done anything to that degree.

But I CAN say that for all other things you ask it will be a LOT easier than anything you did with the s3(or simpler at least IMHO)

For unlocking boot loader, just install the android SDK to your Linux system(VERY easy, just Google "Android SDK ubuntu/Linux). Then after adding the platform-tools dir from the SDK to your PATH(easy instructions found via googling and on the android SDK download/install site) you simply boot into bootloader/fastboot mode(power off, hold power and both volume buttons at same time) then plug into your computer, then open terminal and enter "fastboot oem unlock"(no quotes) and follow instructions that pop up on device and in terminal. Keep in mind doing this OEM unlock command will COMPLETELY WIPE ALL DATA FROM DEVICE INCLUDING PICTURES AND EVERYTHING LOCATED ON /SDCARD PARTITION ;) .

After that it's a walk in the park to fastboot flash custom recovery img (download whichever recovery version you prefer, I use twrp, make sure you get the one specified for toroplus which can be found again easily via Google "official twrp toroplus".. Be certain you are using the NEWEST img which will be 2.7 or above(not sure if newer released yet) so that you do not get error installing a 4.4 based ROM & gapps)

After unlocking bootloader I recommend using the device itself to download the ROM and gapps zips that you intend to install so that you do not have to use adb to push the files to /sdcard..its just easier.

Then you just boot into recovery mode(after install of custom recovery of course) and choose to install your ROM and gapps zips. Reboot and presto.

---------- Post added at 10:37 AM ---------- Previous post was at 10:32 AM ----------

As for getting back to stock, that is ALSO VERY SIMPLE! gotta love a nexus device!

You just go here on your computer, https://developers.google.com/android/nexus/images

Grab a stock factory image package(make sure to get the one for your specific variant, for sprint gnex it is mysidspr aka toroplus) and then uncompress it on your laptop, open terminal in that dir, then just plug device in while in bootloader, and run the flash-all.sh (on my setup I have to do "sudo ./flash-all.sh" then just be patient and DO NOT UNPLUG until FULLY FINISHED (radio flashes might seem to take a long time)

If you need any clarification on anything and cannot find the information in these forums, just let me know. Happy to help out.

And welcome to the exciting life of a nexus device, things are sooo much simpler in these lands! :D

---------- Post added at 10:43 AM ---------- Previous post was at 10:37 AM ----------

Get cwm touch IMG from here

http://www.clockworkmod.com/rommanager

For sprint it is listed as Google Galaxy Nexus(sprint)

Get cm11 snapshot from here

http://download.cyanogenmod.org/?device=toroplus

And gapps are easily found pretty much anywhere, and links are on that last linked site for cyanogenmod.

Enjoy!

And remember, Google Search is your best friend with the Galaxy Nexus Sprint/toroplus tag before anything you will want, Pretty much has official support and downloads for everything :)