San Andreas's side missions are some of the most worthwhile in the series because they hand out permanent upgrades, not just cash. Several are also required for 100% completion. Here is every odd job and what it gives you, with the high-value ones first.
Hop in an Ambulance and complete all 12 levels of Paramedic in one run and CJ's maximum health increases by 50%. This is one of the best early investments in the whole game, because more health makes every later mission more forgiving. Do it before the story gets hard.
Firefighter (fireproof)
Take a Fire Truck through all 12 levels of Firefighter and CJ becomes fireproof on foot: walking through fire stops hurting you entirely. It is a niche bonus, but combined with the others it removes a whole category of damage.
Get into a police vehicle and clear all 12 levels of Vigilante (called Brown Thunder in the Rhino/Hydra variants) and CJ's maximum armor increases by 50%. Paired with the Paramedic health boost, these two together make CJ dramatically tankier.
Taxi Driver (nitro and jumping cabs)
Pick up 50 fares as a taxi and every taxi in the game permanently gains nitrous and the ability to bunny-hop. It is a fun reward and a handy one, since cabs are everywhere.
Pimping (passive income)
Pimping uses a Broadway lowrider (commonly found at the gas station in Idlewood) and works like the taxi job, except finishing it makes your girls pay you rather than the other way around, giving a small ongoing income stream.
The rest
San Andreas is packed with smaller jobs that also count toward 100% or pay out: Burglary (rob houses at night in a Boxville), Freight and Trucker hauling, the Quarry missions in the desert, Valet parking (which earns you the Vank Hoff Hotel), Courier routes in each city, and the BMX, NRG-500 and other vehicle challenges.
The priority order
For maximum benefit, do Paramedic and Vigilante first for the health and armor boosts, then Firefighter for fireproofing. Those three turn CJ into a far harder target before the campaign's tough missions. The rest you can mop up at your own pace, and several of them double as 100%-completion requirements anyway.
Sources