Season 2 is my favorite season, but I think there are some valid points here.
Inca Mummy Girl - ...I just can't get into this episode. It seems very contrived and doesn't make sense. What about the real Ampata? Getting someone to live in your house for a foreign exchange program is more complicated than picking someone up at the train station that say they are who they say they are. And then the mummy girl stuffs hi corpse in the truck? Wouldnt Buffy had smelled a decomposing body? Ehh, just not a good one in my book. The only redeeming thing is when Xander eats the Twinkies and getting to see Oz fall for Eskimo-Willow.
Spike/Dru - I think it was imperative to have Spike and Dru there for Angelus to fall back with. He wasn't just evil, but he had his evil gang too. Otherwise, it would have just been Angelus going around by himself which wouldn't have been as fun to watch or as interesting. We don't have the whole back story of Dru, Spike, Darla, and Angelus quite yet in the show, but when we do, it makes the whole season 2 thing more meaningful: the old gang is back together wreaking havoc and chaos. I loved that about season 2! Plus, seeing Angel and Spike have that tension from almost the get-go was simply fantastic.
Go Fish - cute episode. And I am big fan of any of the episodes that are really tied in with the school or school activities (The Witch, Some Assembly Required, School Hard...) because I was in high school when I started watching Buffy so it was easy to relate. But yes, I also felt like it was an awkwardly placed episode between the serious string of episodes from Passion, Killed By Death, I Only Have Eyes For You, and Becoming parts 1&2. But it's possible that Joss and the writers wanted something more fun and less angsty amidst the terribly serious and depressing events of the episodes around it. Yes, Angel was killing again and Jenny had died, but these kids were still only 17y/o and still wear dealing with "normal" high school problem.