Climate
This is the end of the world, folks. Expect cool temps, rain, and the straight of Magellan´s infamous winds even in midsummer.
Flora and fauna
Seagulls and the occasional seal or dolphin may be seen.
Understand
The cape is accessible by boat, or by a 4+ day round trip challenging trek, the details of which are explained below. On the hill above the cape is a large cross, Cruz de los Mares, marking the the end of the American continent.
The trek is exteremely challenging, with rocky beaches and river crossings which must be timed with low tides. The trek covers remote territory where help could be days away, especially considering waiting for low tides to cross the rivers one of which is still waist deep. For this reason it is not advisable to go alone. Ships occasionally pass in the distance, so with proper signalling equipment it may be possible to signal help from ships which cannot help you themselves but can call for help in case of an emergency.
The path is not well marked, but if you get lost you can always follow the coast when possible -- and paralleling the coast inland otherwise to find your way back.
It is critically important that you look up the tide tables at shoa.cl (http://shoa.cl/) go to Servicios -> Mareas, then get the tide tables for Punta Arenas for the dates you will be trekking. The tide tables for Punta Arenas should be close enough.