As far as I know, there are no “sneaky” ways to use the 64 bit extensions in 32 bit, though some functions on a 32 bit CPU are already 64 bit or more, FPU, SSE and other subsections may work with 64 bit operands.
The only major plus, is that CPUs with 64 bit capability also tend to support the NX / XD bit, for hardware data execution prevention. I’d recommend using the “optout” method, where DEP is applied to all system components and applications except those where an exeption has to be made. Ideally, replace all applications that require exeptions and enforce DEP completely. It is a pretty robust defence against many known or unknown attacks that rely on buffer overflow or similar vulnerabilities to cause execution of code.
The main advantage of a 64 bit OS, is in working with either excessively large application virtual sizes, or with memory larger than 3GB.