This article is from the Baby Proofing FAQ, by Sandra Smith, email@example.com with numerous contributions by others.
When our oldest daughter was that age she was fascinated with the
hearth (a raised brick platform) also. We stopped having fires,
obviously, so we weren't worried about the flammability of a hearth
pad. We used an ensolite foam pad, the kind backpackers used to use
and held it down with duct tape. It was pretty apparent to anyone who
entered our living room that there was a toddler in the house! But of
all the tumbles she took that launched her into the hearth she never
got so much as a bruise. Gradually the hearth lost its appeal, we
removed the foam pad, started having fires again, and found new things
to worry about. But I will always remember the time I walked into the
living room just as she was climbing onto the hearth and she looked
over her shoulder at me and said "no-no. Hot!", laughed, and continued
climbing up onto the hearth.