Midway through the game, you might get access to a swank, top-floor loft safehouse owned by your pal Playboy X. If that happens, try changing clothes while you're there; as you cycle through your outfits, you'll find a new one that's a dead ringer for the clothes worn by GTA III's hero, Claude. Yeah, we always thought the black jacket was more of a leather biker number, too, but there's no mistaking those olive-green cargo pants. It's not the slickest outfit in the game, but if you're feeling nostalgic, slipping on Claude's ensemble can add to the fun of mass destruction.
Above: GTA IV's Niko Bellic on the left, GTA III's Claude on the right. Hooray for historical re-enactment!
The amusement park at Firefly Island is completely shut down and therefore not really worth exploring - unless you want to see some really cool references to earlier GTAs, that is. Visit the mini-golf course sometime, and you'll be able to see miniature versions of landmarks from other games scattered around. These include:
The Ocean View Hotel from GTA: Vice City
The giant chicken from GTA: San Andreas, which doesn't move...
... but which does have a comb that disappears depending on the angle from which you look at it.
There's also the farm windmill from San Andreas' countryside...
... the Watts Towers re-creations from Los Santos...
... and the lighthouse from the old Liberty City, as seen in GTA III and GTA: Liberty City Stories.
Here's where to find it all: