Press R + Z at the opening screen. 
Then, enter the codes.

Bouncy balls            TOOMUCHFUN 
All tracks              THEUNIVERSE 
All characters          HAPPYHEADS 
Iggy's girlfriend       ENTAROADUN 
Turbo mode              2TIMES 
Ice platforms           ICEPRINCESS 
Gooey platforms         GOOEYGOOGOO 
Icy and gooey platforms GOOEYICEPRINCESS 
Level select            JUMPAROUND 
Mix and match mode      SWOPSHOP 
Random bombs            OHMY 
Full turbos             GOBABY 
Fat reckin' balls       TOOMUCHPIE 
Small balls             MICROBALLS 
Non-stop roller ball    NONSTOP 
Turok 2 graphic         2ROKTOO 
Black and white mode    ROLFHARRIS