Well, I'm gonna get my copy of Mario Kart: Double Dash!! in six days (my birthday), and my freind just bought his copy and played it at my house. I've noticed several cool little things in the game. Here's a list of some stuff I've noticed: (Keep in mind that anyone can add to the list. Also, there shouldn't be too many spoilers in here)
-In the first course, Luigi Circuit, behind one of the walls you can see Luigi's Mansion (You know, the good, "A Rank" one that you win after you beat Luigi's Mansion with lots of money).
-On the second course, Peach Beach (which is really Isle Delfino), there are Piantas, Nokis, and Cataquacks (think Gelato Beach) on the track. Also, you can see Daisy Liner (I think that's its name), which is another course that happens to be on a cruise ship.
-Speaking of Daisy Liner, the music that plays in that level is from Koopa Beach, the beach course in Mario Kart 64. (It's also from Super Mario Kart, but I don't think it had that music.)
Well, that's it for now. When I actually get the game, I'll add to the list.