Hints:

Unlimited Lives:
     On  almost all of the levels there is a bubble with an extra life
inside. There is a little trick/cheat you can use to get as many lives
as  you  want. Just go to any level where you know of an easy to reach
life  and  go get it. Then exit the level and replay it and go back to
the same spot and get another life. Just keep repeating that until you
have gotten all the lives you want.

Sonic/Shadow Chao:
     To  get  a  Chao that looks like Sonic or Shadow feed it lots and
lots  of  green  Chaos Drives and/or lots of Green Class Small Animals
(you  can  tell  the  class by what color the border around its little
picture  is).  Then  pet it with a Hero character (Sonic, Kunckles, or
Tails)  until  it stops changing its color to get a Sonic-ish Chao. To
get  a Shadow-ish Chao pet it with a Dark character (Shadow, Rouge, or
Eggman) until it stops changing color.

Hero/Dark Chao:
     Just  pet  the  Chao  until  it  stops changing color with a Hero
(Sonic,  Kunckles, or Tails) to get a Hero Chao and when it evolves it
will  have a halo instead of that ball thingie and be completely white
(except  for  the  parts  that  have  been changed to resemble a small
animal you gave it, of course). Just do the same with a Dark character
(Shadow,  Rouge,  or  Eggman) and when it evolves it will be black and
have  a  spiky ball thingy instead of a perfectly round one. Note that
if  you  give it a lot of Green Chaos Drives or Green Class animals it
will  turn  into  a  Sonic/Shadow-like  Chao as explained above in the
previous hint.

Hero/Dark Chao Gardens:
     To  get  the  Hero Garden and/or Dark Graden just raise a Chao of
the  respective  alignment. The Hero Graden is smaller than the normal
garden  and  is  bright  and  heavenly,  while  the Dark Garden is the
largest  of  the  three  gardens  and resembles a cymetery and has red
water instead of the normal blue.

More Chao Races:
     To  get  more Chao Races just beat all the beginner races. If you
do  you will open up Jewel and Special races. If you have unlocked the
other two gardens you will also unlock Hero and Dark races

Unlock the Green Hill Zone:
     To  unlock a 3D remake of the first level of the first Sonic game
ever  made  collect  180  emblems (so you have to beat the WHOLE game,
completely).

Knuckles Spiral Uppercut:
     Rotate  the  joystick  clockwise  or  counter  clockwise (doesn't
matter) two times. Then push the "B" button.

Rouge's Screw Kick:
     Rotate  the  joystick  clockwise  or  counter  clockwise  (doesnt
matter) a couple times or once then push the B button.

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Bonus Outfits in 2P Mode:

Sonic's PSO Outfit:
     To  get a PSO outfit for Sonic in 2P mode, get all of the emblems
in all his MISSIONS (not stages, there is a difference).

Shadow's PSO Outfit:
     To get a PSO outfit for Shadow in 2P mode, get all of the emblems
in all his MISSIONS (not stages, there is a difference).

Tails's Red Mech:
     To get Tails's red mech in 2P mode, get all of the emblems in all
of his MISSIONS (not stages, there is a difference).

Eggman's Camo Mech:
     To  get  Eggman's camo mech in 2P mode, get all of the emblems in
all of his MISSIONS (not stages, there is a difference).

Knuckles's PSO Outfit:
     To  get  Knuckles's PSO outfit in 2P mode, get all of the emblems
in all of his MISSIONS (not stages, there is a difference).

Rouge's Dark Outfit:
     To  get Rouge's drak outfit in 2P mode, get all of the emblems in
all of his MISSIONS (not stages, there is a difference).

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Bonus Characters in 2P:

Amy:
     To  be  able to play as Amy in 2P mode, get all A rankings on all
of Sonic's MISSIONS (not stages, there is a difference), you will then
be able to replace Sonic with her in 2P mode.

Metal Sonic:
     To  be able to play as Metal Sonic in 2P mode, get all A rankings
on  all  of Shadow's MISSIONS (not stages, there is a difference), you
will  then  be  able  to replace Shadow with him in 2P mode. Note that
Metal  Sonic is not just a shiny version of the normal Sonic model but
an evil mechanized version of him that looks different.

A Chao:
     To be able to play as a Chao in a Chao Mech in 2P mode, get all A
rankings   on  all  of  Tails's  MISSIONS  (not  stages,  there  is  a
difference),  you  will  then  be  able to replace Tails with it in 2P
mode.

Big the Cat:
     To  be  able to play as Big the Cat (a large male purple cat from
SA1)  in  Eggman's  Mech  in  2P  mode,  get  all A rankings on all of
Eggman's  MISSIONS  (not stages, there is a difference), you will then
be able to replace Eggman with him in 2P mode.

Tikal:
     To  be  able to play as Tikal (a female pink echidna from SA1) in
2P mode, get all A rankings on all of Knuckles's MISSIONS (not stages,
there is a difference), you will then be able to replace Knuckles with
her in 2P mode.

Chaos 0:
     To  be  able  to  play as Chaos 0 (a human-ish creature made from
water  from  SA1)  in  2P  mode,  get all A rankings on all of Rouge's
MISSIONS (not stages, there is a difference), you will then be able to
replace Rouge with it in 2P mode.