I would avoid any outing directly after school. Children of 6 are still having difficulty with transitions (home> school-> shops-> granny's-> into/out of uniform-> into/out of bath-> TV on/off---> bedtime, etc). Best to decompress after school either at a park or at home on her own, depending on what her personality dictates. Some like to shake off school with exercise and some prefer quiet and being left alone for a while.
I would make a deal with her before any outing, and shake on it. Then remind about expected behaviour in a sideways way a few times as the outing progresses by telling her that she is doing a good job of sticking to her end of the deal. Thank her as you leave if an outing has been problem free. Don't buy anything to say thank you. Your verbal praise is enough.
If things go tits up, remind her calmly that you had a deal, that it looks as if she is breaking it, and ask her is that what she intends, reminding her of the consequences you have discussed. If yes then leave. If she tries persistetly to engage you in discussion of the thing she wants, complains about you being mean, unfair, etc, tell her that those topics will not be discussed - it is a breach of the agreement to keep on fighting. If she persists take that as a Yes, and leave.
If no (she wants a second chance), and she stops whining/crying/accusing you of meanness, etc. then ask her can she return to fulfilling her part of the agreement within a few minutes as others are waiting/time is short, whatever. Thank her for her effort if she can behave. Proceed as before and remind about good behaviour obliquely by praising a good job if you see that.
Tell her after a tantrum or any episode that has made you leave and everyone is in the car that you will not drive if there is any carrying on there. Wait her out if she is still in fight mode (accusing you of meanness, unfairness, etc, or still crying, whining about mini golf, etc). After a tantrum subsides, you need to tell her that any further tantrumming or unsafe car behaviour (tell her what this is - throwing stuff, kicking your seat, teasing, bothering, smacking other passengers, etc) will result in you pulling over and stopping until she can behave safely. If she goes berserk remind her you cannot drive safely while she is doing that, pull over safely, and stop the engine. Tell her you will not drive while the ruckus is going on and you will wait until she is calm.
If she tries again to engage you on the topic of the treat she wanted or how mean you are, how unfair, etc., tell her you are not discussing that. You are all sitting there waiting for her to produce safe driving conditions. Do not engage with her on any topic she kicks off about, whether the original one or the offshoots.
Thank her for getting herself under control before you head off into the traffic again. Ask her for a deal for the rest of the trip home. Remind her that the consequence of any more unacceptable behaviour will be pulling over again.
To repeat - don't discuss the topic that she is making the fuss about, ever. Stonewall her.
Don't drive angry or distracted. Don't argue or shout while driving. Do a little conscious breathing before entering the traffic.
It is a very good idea to offer an apology if you look back and realise you have been unreasonable or have modeled behaviour you would not want her to copy. Do this a few hours later even if you realise soon after that you were wrong.
Also a good idea to reconnect by means of a hug and an affirmation of some good point of the child's.