A general, quick answer is...
"Nope, no alternative."
You didn't tell us what hardware you need to monitor or investigate.
SPARC?
x86?
Sun-brand?
non-Sun brand?
I'm going to make some assumptions.
<-a-> You have a Sun-branded SPARC-based system.
PRTDIAG is (until very, very recently in Solaris 10 or Nevada)
only for SPARC systems, not x86 architecture.
<-b-> Your system is something 'small', such as a workstation or a V440.
Different platforms have different capabilities.
They differ with the complexity of their board-level designs.
Systems without ALOM may not even have the chip-level design to report anything.
(try PRTDIAG on a SB100, and you will see nothing useful)
More complex systems, such as V480/V490 or V880/V890
have their PICL daemon and can provide information to many outputs.
Those systems have SEEPROMS throughout the chassis
and communicate tons of information.
ALOM-capable systems log information to the admin circuits
in addition to their Operating Environment.
Sunfire Midrange Systems (SF3800 through SF6900) gather information
at the platform shell as well as up in the domain shell.
Again, without knowing which specific model you're discussing,
it will be difficult to give any useful advice.