I would echo the point about general exploration in the northern part of the map. This game has a massive hoarding element which I'm sure you already know. Pick up all the ammo and health items (like stimpacks, radaway, etc) that you can and always be on the lookout for new weapons. If you come across a new weapon or type, try it out. You may like it better than what you have now. I'd also suggest lockpicking and /or hacking perks. These will help you attain more resources like ammo, caps and other things you'll want more of.
When you get to it, don't get too caught up in the settlement building (unless you like it of course). I haven't personally discovered any big reasons to expand settlements apart from having other places to stash your things or a safe place to keep a set of power armor. Those points get diminished further when you factor in fast-travel, as you can just fast-travel back to your main stash spot.
I personally don't use power armor much at all, but it can really be a powerful tool. The game doesn't force you to use it though, so if you like a stealthy playstyle, it's still very viable. It's hard to lose if your enemy never sees you coming.
One last thing, if you find you're using a certain kind of gun (like automatics or rifles) there are perks which greatly increase your damage with these weapon types. If you couple in gun-nut with these increased damage perks, enemies will start going down really quickly. I personally put a lot into Rifleman, as I like snipers, semi-auto rifles and shotguns (yes, they're counted as rifles in this game). With this strategy I have long range covered with snipers, mid-range covered with combat rifles, and close range covered with shotguns. Be aware though that if you mod rifles or shotguns to fire differently than your damage perk (an automatic shotgun when you have rifleman for example), your damage perk won't apply.
But yeah, there are plenty of quests that you can do before going into diamond city. I put at least 40 hours in before I even made an attempt to head over there. I've noticed that you can find random quests just by exploring random places and buildings and by talking to random people.
Most importantly, have fun! Play whatever way you find most comfortable. This game is deep enough that you can make multiple playstyles viable.