The only other time we see Cas watching television (as far as I can recall) is when he was watching porn in Caged Heat, and judging by his fond smile here, I doubt that’s what he’s remembering. The fact that he even knew how to work the TV well enough to use it on his own that time is also telling.
I think this fills in a few of the missing scenes we don’t get to see, the little domestic things Team Free Will does in between the big showdowns. There must have been times Castiel stayed with the Winchesters to watch over them, “just in case.” Nights spent in cheap motel rooms with the boys, tense in the hours before battle, but with an air of camaraderie that is familiar to Cas from his days in the Garrison. Nights where there was nothing better to do than watch crap TV on a grainy screen. Maybe they would watch action flicks, or the occasional Doctor Sexy marathon - “it’s not like there’s anything better on,” Dean would insist - while trying to explain some of the more subtle nuances to Cas. Then Sam and Dean would crawl into bed and fall into an uneasy sleep. Hours would pass, the light of some late night infomercial illuminating their sleeping forms, and Cas would watch the steady rise and fall of their breathing before falling into a quiet meditation of his own.
Maybe it’s nights like these that Cas recalls when he watches these infomercials so fondly now. And I have to wonder if Cas’ idea of a perfect last memory wouldn’t be so far off from Bobby’s.