When Barry is doing physical exercise on the salmon ladder at super speed, Caitlin asks Felicity how often Oliver does that. Felicity responds by saying "Almost every Wednesday". Wednesday is the day of the week that Arrow airs on the CW.
Lyla refers to Barry as "Speedy". Oliver assumes she is asking him about Roy. This is a reference to the DC comics character of Roy Harper, who was known as "Speedy" in his early career, before assuming the title of "Red Arrow" and eventually, "Arsenal". In the TV series Arrow (2012), Oliver calls his sister, Thea, "Speedy". He has also called Roy Speedy in the past.
Quentin Lance mistakenly calls Barry "Bart Allen". Bart Allen is Barry's grandson, and was also known as Impulse early in his career. However, this could also be a nod to Barry's full first name, "Bartholomew."
George (aka Digger) Harkness is a comic book villain who made his debut in Flash #117 (Dec 1960). Originally from Australia, he moved to America as a child and used his skills with a boomerang for demonstrations, all the while taking people's money. This grew into larger crimes and he eventually ran afoul of DC's top heroes.
Cisco calls Oliver's hideout an "Arrowcave" and asks if there is an "Arrowmobile", much to Oliver and Felicity's chagrin. In the years when the Green Arrow comic book was written as simply another copy of "Batman", Oliver Queen did actually have an "Arrow Cave", an "Arrow Car", an "Arrow Plane" and other arrow themed vehicles and gadgets, much like his Gotham City counterpart. The cave still appears occasionally in the modern era comics.