ZenonSA is a simple, no-frills program that can find surface areas and volumes of rectangular prisms, triangular prisms, cylinders, cones by cone and slant height, and regular prisms and pyramids. There's no super-bloated MFC interface to crash on you, and ZenonSA will most likely work on an HP95LX. (Note: the binary will not work on an HP95LX, but a small change to the source code (enabling 8087 emulation) will fix the problem.)
Like all Zenon software, ZenonSA is under the GNU GPL, so you can tailor it to your needs and share it with your friends, unlike most other software, where you have to wait for some multi-billion dollar sloth to fix things, and the software police come after you if you share their stuff with a friend.