Second Ship?
Daisy's Cruiser has been in the water off the coast of Yoshi Circuit, but several people have e-mailed me reporting a sighting of another boat off shore. It's unclear as to what this other ship is supposed to be, or whether it's just here for detail. Either way, it's a neat to find another boat floating around.

Pretty Poster
Here's billboard that I didn't notice at first... Along the straightaway and just past the finish line, is a pink sign that reads "Peach Grand Prix". Seems as if practically every character has their own Grand Prix....

Have a Good Year...
Floating high in the sky on Luigi Circuit, is a giant blimp. It travels back and forth around one of the corners, and has 'Luigi Grand Prix' written on the side of it.

Luigi's Mansion...
On one of the turns of Luigi Circuit, you can actually spot Luigi's house somewhere in the background. I played through this track a few dozen times before I noticed this huge house. It's kinda hard to notice a giant building when you're concentrating on speeding around corners while avoiding an insane barrage of items.

Cheep Intro...
Normally when you begin your game, Lakitu appears with a traffic on a pole. But occasionally, he'll randomly have a Cheep Cheep fish on the end of his line. There are actually two different Cheep Cheep animations - One where he brings the fish up, and the other where it falls off his line and makes a splash.

Bowser's Boat...
There are a number of neat little details that can be found outside of the track walls of the Baby Park track... One of these things includes a body of water, with a sign marked "Bowser's Ship". This boat looks a bit like a Doom Ship from the SMB3 era, and it's docked right here, just barely visible for us to spot.

Turtle Tent...
Another thing from the Baby Park course, is a giant Paratroopa on the ground... Not a real Paratroopa, but a mechanical one. It looks as if there's a miniature tunnel at the front, meaning it could be some sort of theme ride or event house. The Paratroopa's eyes scroll from right to left, and the wings even flap.

Doggy Style...
Something else can be found in the background of Baby Park... There are a few pairs of Shy Guys scattered off in the distance, but you can also see Poochie the dog from Yoshi's Island (Mario World 2). That little pup is really hidden well, and he was a really neat find.

Stars & Shrooms...
Since you're usually blasting through the Daisy Cruiser so fast, you wouldn't normally notice something like this... But along the sides of the ship in some areas, are repeated patterns of Stars and Mushrooms.

Traffic Jam
At first glance, the vehicles traveling on the Mushroom Bridge don't appear to be anything special. But a few of them have some details on them...
For example, the bus on the left has 'Peach Beach' right on the front of it. And this delivery van on the right has a "P-Wing" symbol, one of the ultimate powerup items from Super Mario Bros 3.

Truckerpillar
The largest vehicle traveling on the Mushroom Bridge is by far this big green transport. The machine itself was no doubt modeled after the Wiggler caterpillar, a creature from Super Mario World. This truck even wiggles along as it travels, just like a caterpillar.

It's a Bird - A plane - No, a giant freakin' mushroom..?
There's a giant blimp flying over Luigi Circuit, and Mario Circuit appears to have a gigantic mushroom balloon. I hope I'm not the only one who's seeing this stuff... Just to make the race more interesting, wouldn't it be fun to pop that sucker with a BB gun and watch it interfere with the racing?

Vessel off the Port Bow!
Daisy's Cruiseship might be a racetrack of it's own, but the boat can also be spotted from a few other courses as well... To the left we can see Daisy Cruiser from the coast of Peach Beach, and in the right shot we can see it further in the ocean from the Yoshi Circuit track.

Aside from all of the little tidbits, Mario Kart Double Dash is also home to plenty of ads and billboards. These things are scattered throughout the racetracks, and some of these signs have neat things printed on them.
I've only covered a few tracks so far, and I'll post the rest in a future update.

Luigi Circuit
On the left we can see a sign labeled"SHOOT!" with a red koopa shell as a logo. Also there's an ad for "Mario Racing".
The right screenshot shows "Luigi Grand Prix", and also "Luigi Tires".

Peach Beach
To the left is a simple ad for "Mario Racing", with the Sprite from Mario Sunshine.
On the right is a billboard right before the starting line. This ad is for "Delfino Fruits", and the color and look of the ad make resemble that of a cereal box or something...

Mario Circuit
Here's a whole row of ads, found on one of the big curves of Mario Circuit. These ads read "Super Mushroom", "SHOOT!", "Koopa Kart", and "Mario Racing".