Hi John,
+1 for Justin’s idea. I’ve done this myself and it’s cheap, easy and quickly satisfying.
If you want the project to be more like a “flashing neon” sign, you can snip off the battery packs of the string lights.
Then attach these to the business end of a SBC of some sort. (Micro:bit if you have it) No soldering required and simple easy code to flash the lights on and off.
Of course Arduino, RPi W or the like will work as well. These can be powered by cheap USB phone chargers. (Any will do once they output 5V). Not sure about the US, by in Ireland, we have “Euro Shops” that sell cheap stuff…
All materials mentioned can be purchased here (except SBCs) - are reusable for other projects afterwards.