Some classics. The first is called Marlon Brando or The Name Game. You start with the first person saying a famous person (e.g. Marlon Brando). The next player must come up with another famous person whose first name begins with the first letter of the famous person’s last name (following B for Brando they might say Bruce Springstein). The next person needs someone with a first name beginning with S. And so on, round and round.
If both words of a name begin with the same letter (e.g. Marilyn Monroe) the order reverses. And if a player can’t think of anyone, or names someone previously thought of, they lose. And must exit the vehicle immediately in shame.
The second is a tricky letter word game, which I think is called Ghosts. This is played as follows:
The first person says a letter, e.g. B
The second player adds a letter either before or after the first letter. The rule is that these letters must be part of a word (occurring consecutively) but cannot complete a word. So adding an E to get BE loses, but making RB or BB or BU would be fine (HERB, HOBBLE, BUST), and BX would probably get you challenged (see below).
The third player then adds a letter according to the same rules, and so on for the fourth etc. A player who completes a word loses the round. However after the second letter, players also have an additional option. If they think the letters presented to them form no real word, they may challenge. If they win the challenge the player who added the last letter loses; if the challenged player can present a word, then the player who did the challenging loses.
Examples (rob, sarah and teri play):
rob: C
sarah: CA
teri: SCA
rob: ESCA
sarah: ESCAP
teri: ESCAPE Duh! teri loses (but what other word could she say?).
rob: D
sarah: DZ
teri: ADZ
rob: challenge
teri: I was thinking of “adze” (medieval woodworking tool). rob loses.
This game obviously depends on your vocabulary, but also has opportunities for bluffing and guessing if others are buffing.
Thirdly, you can do charades with sound effects instead of gestures (speaking actual words is still not allowed, except perhaps to say if it’s a book, film, etc.) For instance “Aaargh! Aaargh! AAARRRRGGGHHHH!”= Scream 3.
Truth or Dare might be another possibility. If you can think of dares that don’t involve the driver closing his eyes.
I guess you don’t want to play anything too raucous for safety reasons, but if you don’t mind crashing you could try a few drinking games, such as the game in which players take turns naming well-known euphemisms for sex. (“sex” is not a euphemism for sex.)