Depends on what you mean by alternative?
Are you asking for an alternative producing the same effects as a marijuana high?
Or just an alternative substance altogether?
Spice is suppose to be a complete mimic of marijuana as far as high goes.
I myself have tried a few different spices, some worked, some didn't.
All depends on the brand.
I've actually found spices that are better then marijuana high somehow.
It's almost like getting high for the first time all over again (when using spice) & such.
Will even produce the munchies.
But that's not to say it's healthier, because it's not.
[edit]
Brand that worked for me 100% of the time: MR. HAPPY.