Grenades in Weapons Stash, Satchel Charges in Weapons Stash, Unlimited Rifle Ammo

One Hitman list completed, Two Hitman Lists completed, All Hitman Lists completed

All over Stilwater

Grenades and satchel charges are nice to have, as is unlimited rifle ammo, however you can get through the entire game without ever chucking a pineapple or laying a satchel charge. The unlimited rifle ammo is nice, but if you already have unlimited pistol, SMG and shotgun ammo, it's just overkill.
Knocking off all of the Hitman targets will take a good amount of time as most targets require some bit of set up. Some may have you doing nothing more complicated than dialing 911, waiting for the ambulance to arrive and killing the EMS tech, while others will have you obtaining a 5 star wanted level so that the FBI comes after you. Whatever target you're going after, make sure to select them from the target list first, so that the Hitman icon shows up in your map when you get close to them. Also, even when you don't have a target selected, be watchful for the icon to show up on your map, and when it does, head over to it and kill your target. Each killed target gives money and respect, so you're not wasting your time by taking out targets, but at the same time, the benefit you get doesn't quite match the time spent.

Everything Else

Everything else on this list is just gravy as they don't affect the game in any real way. Clothing, mechanic and crib customization discounts are nice, but by the time you get all of the other activities completed, you'll have so much money that a discount isn't really necessary. Plus, as you start doing missions you'll get cash for completing the mission plus $500 a day for every neighborhood you take over, never mind the money you'll start bringing in when you can invest in businesses. A discount on ammo is nice, but you'll have unlimited ammo for most weapons, so you won't need to buy any.

The reduction in notoriety brought about by completing activities helps, but in most cases, when you're knocking off a Ronin casino, or taking on a Brotherhood stronghold, they're going to be pretty pissed at you for the entirety of the mission, even with these bonuses. As long as you don't have an itchy trigger finger and can resist the urge to shoot every rival gang member you see outside of missions, you can get by without these bonuses.

Finally, the Pepper Spray and Chainsaw are included in the "Everything Else" section because while fun, they don't give anything over the other melee weapons, especially the katana. Fun for a while, but why spray someone with pepper spray when you can spray them with bullets?

Pepper Spray, Chainsaw

Crowd Control level 3, Crowd Control level 6

Marina
Crowd Control has you protecting a celebrity as they get harassed by over eager fans. Your job is to rake in enough money by taking out fans before the time runs out while at the same time keeping your client's annoyance level down. When you grab a fan, be on the lookout for blue arrows as chucking the fan into the blue arrow will net your more money. Co-op can be useful here as even though the dollar amount goes up, in later levels, having one person chuck fans while another swings away with a baseball bat is key. On the final level, as soon as fans start using machine guns, pick one up and just mow down the crowd. You can't kill your client and there's no penalty for killing civilians, however you can rack up a ton of cash in a short period of time just by laying waste to all of the demented fans.

5% Clothing Store Discount, 15% Clothing Store Discount

Crowd Control level 3, Crowd Control level 6

Suburbs
Nothing new here, grab fans and throw them into comical yet deadly environmental elements

5% Mechanic Discount, 15% Mechanic Discount, Demolition Derby Vehicles

Demolition Derby level 3, Demolition Derby level 6, Demolition Derby level 6

University
Demolition Derby is a death match for cars. Take that Mater! In between rounds you're given derby points that you can use to pump up your cars, and use them you should. Lay on the nitrous to slam into cars with greater ferocity and don't be afraid to mash cars up against the walls over an over again. Do enough damage before the time runs out and you're on to the next round.

5% Ronin Notoriety Reduction, 15% Ronin Notoriety Reduction

Drug Trafficking level 3, Drug Trafficking level 6

Marina
Same rules apply here. After doing this activity, the Ronin will still hate you, just a little less.

Vehicle Delivery for $1000, Free Vehicle Delivery

Escort level 3, Escort level 6

Red Light District

Why you'd pay to have a vehicle delivered to you when you can just steal one off of the street is a mystery, but if you really need to drive your pimped out firetruck and aren't near your garage, this is the way to go.
Escort is just as viable in single player as in co-op, however it's more fun in co-op, but only if you drive and your partner has to pleasure the client in the back seat. Otherwise it's kind of demeaning. If driving, avoid the news vans at all costs and get the client where they need to go. If riding in the back seat, work that ho-ing mini game until you, er, hit the jackpot so to speak.