Phil turns 81 in this episode -- assuming that this episode takes place in 1998 (the year it was produced), that would mean Phil was probably born in 1917. And assuming that he did end up dying at the age of 91, he would've died in 2008.
It also means that, if Phil did end up dying at the age of 91 in 2008, Arnold (who's nine during the events of this episode) would've been around 19 when his grandfather passed away.
Prior to seeing Dr. Stieglitz in this episode, Phil hadn't seen a doctor in 25 years.
Dr. Stieglitz states that he has been practicing medicine for 81 years, meaning he would be around 100 years of age.
Phil confirms that Arnold is his and Gertie's only grandson (describing him as his "only begotten grandson").
It can be assumed that Arnold is also their only grandchild, if you take into consideration that Miles, Arnold's father and Phil and Gertie's son, was presumably an only child and the fact that Arnold himself is an only child.
Gerald has a pigeon coup on his roof.
When Phill wakes up in bed after he thought he was in Heaven, he says "Oh no Oskar's here, this must be the other place". The "other place" is referring to Hell.
Another reference to Hell would be in "Ghost Bride".
The montage of Arnold and Grandpa having a good time is similar to the montage in "Eugene's Bike".
The movie that Arnold and Grandpa see, You Only Live Once, has a scene which resembles one from Harold Lloyd's Safety Last!.
The song Gertrude sings during breakfast and during night is "Bury Me Not on the Lone Prairie" made famous by the legendary Johnny Cash.
