Obviously you are going to drain & refill motor oil. AC deco 1dexos 5w-30 (6qrt) & delco or Napa gold filter.
Even before that I did a drain&refill on the trans with a new filter/seal/pan gasket kit & Delco Dex6 synthetic. The trans needs all the help it can get & this dex6syn is the business. Drain & refill the rear diff. Front diff & transfer box if the vehicle is equipped. Are the front axles or gearbox leaking fluid? That means they are running low anyway. Get the old restricted fuel filter off of there ASAP before you burn up the fuel pump. (If your truck has 8th digit 'Z' in the VIN you will find a serviceable fuel filter) Replace it with Napa gold unit. New 100k iridium delco spark plugs, fresh set of wires, new intake air filter YESTERDAY... Not when you decide to spend $300 on a tube & 3 screws. Purge the old brake fluid out. Flush the power steering lines out & refill w synthetic fluid & 1 full unit of Lucas PS conditioner per GM TSB. Replace the PCV valve. Flush the coolant & heater core. What else? Fix anything that is creating trouble codes i.e. Check Engine Warning, clean the MAF sensor, inspect brake pads/rotors, inspect the front end & make sure you aren't about to lose a front wheel due to faulty ball joints or wheel hubs, make sure the front tires are wearing evenly & it doesn't need a wheel alignment. Does the driveshaft(s) need new Ujoints? Does the slip yoke CLUNK need addressing? Is the intermediate steering shaft bumping & clanking? Do you hear any POP or CLUNK sounds driving over bumps & dips? Does your truck have provision for removable cabin pollen filter? If not, then you need to look up the cabin filter mod. I was breathing dirt, fungus & gym sock odor for months before I figured this 1 out. 2002 you may be good so change the cabin filter. Install new wiper blades. See if the truck shakes or pulses under hard braking. Remove all of the lug nuts & retighten by hand so that you can change out a flat tire. Do you have a spare and check the air pressure. Make sure none of the bulbs are burned out. Buy a spare brake switch for when that goes out. Check the drive belt for cracks. Does the parking brake hold?
Do some research for the common issues and find related Tech Svc Bulletins for fast fixes & easy repair before little problems become large PsIA. I did at least 1-maintenance item every week for over 2-months before I took a break to just enjoy driving the truck. Everybody should have some level of paranoia just buying & jumping into somebody else's junk. These trucks have the most TSBs of any GM vehicle ever manufactured; would be wise to at least be driving with all fresh fluids & filters from the get-go. This espc holds true when past maint records are non-existent, incomplete, undocumented & so forth.
If you are hauling stuff & pulling trailers, new drive train fluids becomes immediate concern. I promise you the PS fluid is black & no longer lubricating or holding a cool temp. I HATE replacing power steering pumps. Your truck is most likeLy running on the original fill differential lube and it's been sheared 10-ways to Sunday. You will love the smell when that rear end cover comes off. newest release trans' Chems have the ability to operate at high temps before shearing & losing viscosity. Dex6 has lower viscosity by-design to operate well while cold/ @ colder climates as compared to old factory-fills, blends, formulas & so forth.
to the forum. As for tune improvements, good previous advise. As for leveling when towing, IMO the most effective and safe way is a EQUALIZER INSERT with removable/adjustable SWAY BARS. Don't forget the brake control if your trailer has brakes. 
