Any question answered...

hire cars - suffice to say you need one, i think we just went with good old AVIS last time. just picked up / dropped off the fucker at the air harbour and off we went - got a little hatch back, does the job. you want something that's reliable and moderately comfy considering the mileage you'll be racking up. make sure one of you knows how to change a tyre as some roads can be in a bit of a parlous state. maybe ask for something with bike rack mounts.

for mountain biking i'd recommend hooking up with one of the many MB tour groups that operate off of long street (i imagine) - sure they might be a bit touristy but you really should avoid twatting down Table Mountain on your tod. Your guide will know the least death-y routes and there's safety in numbers.

http://capemountainbikingtours.com/ - cant vouch for them but they come up first in google. i'll ask my mate who's into the scene there if he has any pro-tipz.

as far as LOLS are concerned - most of the touristy stuff is actually worth doing and you don't need to be off your tits to enjoy them.

In CT id recommend variously:

Cable car up the mountain - tho check the weather (if you're feeling brave you can hike back down - again, weather permitting. not a year goes by when some soutpiel doesn't kark it from hypothermia or twats off a cliff because clouds)

a drive along the coast to simon's town / kalk bay / boulders beach (penguins!) / cape point and back.

climb lions head and watch the sun go down with all the white folks.

district six museum

the mount nelson hotel - a vast, pink monument to colonialism

the labia cinema - for the lols, obvs

Franschoek / Stellenbosch. Sure it's wine central but it's fucking pretty

Chapman's peak drive. extra points for doing it by steering with your knees only.

sandy bay - nudist beach... if you're into that sort of thing

a braai, if you can get invited to one (most sundays - just knock on someone's door)

avoid:

the waterfront

shark diving

township 'tours'

africa cafe / mama africa / anything with the word 'africa' in it.

most 'curios' - very few of them are made locally, especially the tat you get harangued into purchasing at Greenmarket Square etc.

being a dick if someone asks you for money for 'looking after your car' - a lot of people are living in the worst sort of grinding poverty with few prospects. sparing a few quid for what is arguably low-key blackmail is really neither here nor there for rich tourists like you and me.

have fun.