Unfortunately, it is not a "one size fits all" to determine what you should/shouldn't let a 12 yr old do. Maturity, past choices, the area you live in, and your personal beliefs are going to drive most of what you agree to. Here is what we have done with our 6 kids:
Mall hanging out - never. It isn't even allowed at most malls anyways. It is unproductive, unnecessary, and unwarranted.
Park - during daylight hours only, and checking in on occasion, and I need to know who is hanging out with them.
Phone - I always have access and checked frequently. No snapchat, kik, or other apps that allow instantaneous deletion of pictures and texts are allowed on the phone. I also control data times with parental controls so no phones after 10 pm.
Sleepovers - I have a personal hate of sleepovers. Just my own personal thing - I think they are stupid. However, my kids have certainly done their share. My rules is that one parent must be home the ENTIRE time, and I ask the parent in PERSON (never by phone) to verify that someone will be there all night. I also ask what that families curfew is and if it doesn't match ours, I may not agree.
Moving from one destination to another - another pet peeve of mine. If my kid asks to go to Johnny's house and then leaves to go to Tommy's house, I better get a phone call.
Girls - no dating until they were 16. Group boy/girl things only if chaperoned (even if it means sitting in the back of the theatre at a movie).
Yep, I sound like the strictest, meanest mom in the world, but I can't tell you how many times my kids thanked me for being so strict. So many of their classmates (I don't say friends, because they largely hung out with kids who didn't get in trouble) got minor drinking tickets, curfew tickets, flunked a class, got in trouble with the law, did drugs, got pregnant, etc. I have two 19 yr olds currently in the military and if they had gotten in trouble at all, they very well could have been restricted from their being able to do this. My two oldest daughters work at places that require comprehensive background checks and can only work those jobs because they have literally never been in any trouble.
Great childhoods stem from having loving families, spending time together, doing things they enjoy like sports, church activities, boy scouts. I don't know anyone who said "I had a great childhood hanging out at the mall."