Itd be ideal if I could get them to scan, where one or two leds at a time in sequence down the string.
I'm afraid that will not be possible. This kind of lights is usually made of RGB LED which are all in parallel , so all are of the same color at any time.
Have a look at this kind of LED string
. It has "8 memory modes (Combination, In Wave, Sequential, Slo Glo, Chasing, Slow Fade, Twinkle,Steady On)" which may be exactly what you want.
if id be better served with a more customized approach.
Entirely possible with WS2812 based LEDs, but probably not within the range of your possibilities (need microcontroller plus programming), Plus in total likely to be more expensive than the above linked strip. On the other hand a good exercise to start in the world of programming microcontrollers. If this is for Halloween, the time frame may be a tad short to get you going from scratch using his approach.