An On30 engine would be absolutely perfect!
Real narrow gauge equipment is used in most parks.
Disney, Knott's berry farm, Cedar Point, I think 6 Flags, and the reopening Kings Island all use small NG locos. If you stick to an 0-4-0 Porter you could likely go down to 25"-30" curves if you can find them (Piko lists they made/make some really tight 12.5"ish ho curves, but I never actually found any for sale) (what a summer to want to relaunch Kings Island eh?; right when they're ready to go, they can't.)
I'd look at On30 offerings for a good park RR vs a "train ride" in N or HO or even OO or S.
Many of the parks used narrow guage anyhow. I only saw the mini trains at metro zoos and a small few roadside RR attractions here and there. Fun to ride, yet so disappointing to know the steamer had a diesel motor and the diesel was a 4 stroke gasoline motor. I enjoyed the ride, but didn't really enjoy those trains too much 
. The gas belonged in an interurban and the diesel belonged in the diesel, any 5 year old knows that! 
I would have enjoyed a donkey cart ride just as well I suppose 
If it was a miniature donkey vs "fake"mini train, I'd grin more at the little oat powered machine for sure ! 
(I was a little rivet counting train snob sometimes..Still went every time, but whined the whole time about "rivits" 
)
The On30 stock is smaller than most O, yet not too small for O figures, etc. On30 is O scale 30" narrow gauge track {uses HO track ok, but true On30 has bigger/less ties} (similar "On3" is O scale too, but on 3' track, so trucks are too wide for HO/On30 track) (to throw in a little more confusion, kit bodies are sometimes 1:43, 1:45, 1:48, but I think 1:48 is most common.)
Premade On30 is out there, but takes more digging to find and is usually pricey outside of Bachmann. Bachmann has the easiest offerings to locate, but there is more and better than that too. Most kits are bashes on Bachmann equipment doners (ho or On30 equip)
You want a "carpet drive" for the open air trolleys. Kits for those open ones go back decades (carpet drive is a motor(s) inside a truck between wheels vs "on it") Folks were more inclined to build them half open/half enclosed to hide a motor... or there was a big square in the center of the seating.
There are a few examples of open air coach cars by folks here where pretty much all they did was cut away the window panels and doors. (I'm pretty sure I just saw an On30 open coach kit on D-bay too) OR folks added seating and or roof to a gondola (both done prototypically for excursions )
Circus cars and equipment would be pretty much at home on a surrounding mainline to some extent imo. But mostly just ng passenger coaches for the park RR. ( A calliope on a flat, or in a semi-open box car to give it a roof would be "passible" imo. ... so might a diner car(s) , maybe each with a food theme, cotton candy car, balloon car, taco car, sausage car, soda car, etc, etc. (Kline made a caliope wagon load, & I just saw a "last" one of those on De-bay too.)
Wardie Jay kits are the old circus car kits you'll want to look at too (much of this is written killing time until that name was recalled, lol)
I guess it really depends on the style of park too. Some are much more like a quiet shady villiage, others like a huge parking lot full of mechanics, lights, and noises. Even the crowds impressions, reactions, and moods are highly driven by the style. For the latter, "chase lights", chase lights, and some more chase lights to drown out those other chase lights... repeat. (I worked following carnivals and fairs for private food concessions in my teens; a tamer life than the "real" Carnies but still a blast with sights you can only imagine that stick around forever 
)
So, other thoughts are street vending carts , decorated diner, etc.. A picnic area and gazebo, maybe with a band. Some small "old west" buildings for knicknack shopper area among winding paths and trees.Skill games, cotton candy, pocorn, ice cream on a Midway strip.
The feeding spots for the colorful carp or other animals are other places I have fond memories of at amusement parks too. I arrived early and hit the big rides then relaxed and let saner opportunities steer the day, maybe hit some again near close, but near always a planned all day affair. (short days where spur of the moment trips)
