Retro Grouch 

I don't think you should overthink this. What do you think reasonable for people to expect from you? Don't think you have to provide services that you aren't experienced in providing.

"First, do no harm."
If you haven't done some pretty significant bike wrenching, don't think that you suddenly have to gear up to do that.
If you aren't first aid certified, that's what 911 is for.

If a bike or rider isn't able to complete4 the course for any reason, you should be equipped to load them both up and drive them to the finish. A little cold water is always a good thing to have available, but you'll probably need less of that than you are anticipating.

sean.hwy

Bike hitch
ice chest full of ice ( injures, clean ice for drinks )

Herzlos

Is the ride being organised by a cycling club or group of some sort? If so, I'd ask if the group has any old/spare parts they can donate or let you borrow. Most of us will have some tires, tubes, pedals, chains and brake pads that aren't needed. Even older/worn stuff will get someone to the finish line. Beyond that, a set of tools, energy bars, water and towels and you're sorted.

I'd imagine the bulk of your breakdowns will be flat tyres or snapped chains though, and most of your stops will be riders who are struggling.

Ironfish653

There's a lot of different levels of SAG support, depending on the event.
​​​​​​For a one-day, public entry event like OP is describing, SAG is the broom wagon, nothing more.
.It's not the TdF, and you're not the "Team Car"
​​​​​​Unless you are driving a Velofix van, you're not a mobile mechanic, either.
If SAG stopped to do repairs for every breakdown (especially on a rando public entry event) then, by the time you got two or three in, you might have people waiting half an hour for you to get to them
Even on the small-group, multi-day tours I've done, if you had any issue bigger than a flat tire, SAG would just pick you up and run you up the road to the next rest stop / rally point, or wherever the actual mechanics were set up. .

On private tours like that, the SAG crew does more, like providing intermediate rest stops, and carrying extra rain gear/ jackets, but the primary goal is to get everyone to the end of the route by the end of the day.

Wildwood 

Tip bucket.
(nothing is free these days)

tempocyclist

Haha! I never knew it stood for that, thanks. 🙂 I always thought it was "sag" as in "you're sagging behind" and need a repair or help. Like the broom wagon sweeping up stragglers.

martianone

not sure what your SAG vehicle is, suggest a bike rack or other way to carry a bike or two.
cell phone contact info for the ride leadership - as well as local police, ambulance contacts, etc.
usual high visibility shirt or vest for yourself -
a large umbrella

seypat

If it was a SHAG wagon, you'd need plenty of spare tubes!

Bald Paul

UPDATE:
I only had two calls for repairs on the ride - both flat tires. The first person didn't have tire levers with him (although he brought a spare tube!) so that was a quickie. The second was a bit tricky. Rear flat, had tube and levers, but he had just had his bike serviced at his LBS. The QR lever was positioned against his chainstay (carbon fiber bike) and must have been closed with a hammer! He called for a SAG after giving up trying to get it to release. I couldn't budge it either, and wasn't really willing to start prying against the chainstay to try to get it loose. Luckily, I had brought a patch kit. Slid the tire off, pulled out the tube, located the hole, patched it, reinstalled tube and tire, and got him on his way. He's going back to the shop today to get them to release the QR.

The only other disaster was right at the start of the ride. It began downtown, and the riders were to follow a van through the downtown streets. Just as they approached an intersection, someone in a car ran a red light and T-boned another car (luckily, no riders were involved) and ended up on the other side of the intersection. The van and part of the field were able to get through, but fire and police were right there, and blocked the intersection off, detouring the rest of the riders.
Since the van was supposed to lead everyone out, the course arrows were not used in town, so half the field got totally lost. I felt like a cowboy in the old west, trying to round up the stray cattle on a drive. As far as I know, we didn't lose anyone, but if you see a lone rider looking for arrows painted on the road in NC today, let me know.

p.s.: next time you're on a SAG supported ride, please thank the SAG driver(s), even if you didn't need them. It's not as easy as it looks. I ended up driving almost 70 miles to cover the 33 mile route (first loop very slowly, backtracking for the flat issues, second loop as a "broom sweep" to look for stragglers). I did snag a cookie at a rest stop.

Garfield Cat

The organized ride does have a SAG food and drink wagon. That way you do not have to carry liquids and food. Maybe except for yourself. Just don't spill the coffee.

Second, before the ride, your SUV needs to be emptied out of your personal "stuff" so that nothing gets in the way when you're trying to help fix something.

Third, if you have an extra set of platform pedals, bring it. Sometimes riders will have issues with their cleats and or pedal system. Those kinds of issues will render the ride "finished". This means you need to carry that special wrench to unfasten that nut. If somebody tightened that nut too much, you will need to bring a wooden block to act as a wedge for extra force. Watch how a bike mechanic does it in his shop.

Fourth, if its a particularly hot and humid day, the rider may be sweating a lot. To cool a rider off, the first thing is to remove the helmet while you fix what needs fixing. If sweat reaches their eyes, bring paper towels and then insert one folded towel inside their helmet near the front where the sweat drips down the forehead into the eyes.

Fifth, its about you and the rider. You are going to be the person with that calm and confident personality to give assurance to the rider that everything will be ok.