I'm used to rent a car when staying at CKS. The biggest disadvantage is, that you shouldn't drink when driving. Thai police doesn't really care about speeding, but they care a lot about DUI. The second biggest disadvantage is, that it's not easy to find a parking lot in some areas. And when it's really hot on daytime, you don't want to walk around much.
Of course a dinner at Chaweng is doable. All in all you will need about 45 to 75 minutes from CKS to Chaweng. Sometimes there are some slow cars on the road and it's not always reasonable to overtake them. And when it's raining, it will take more time as well. And it's dark when it's dinner time. Driving in Thailand during night time - especially on the smaller roads close to the CKS - is maybe not fun for everyone.
If you want to explore Chaweng - the beach bars etc. - I would highly recommend to stay at a hotel at Chaweng like the Vana Belle. It's much less calm and less romantic, but it's easy to join the party people and it's no problem have some drinks there, because you don't need to drive. On the other hand there are better party locations in Thailand than Samui, i.e. Phuket.
If you drive around the island you will need about 1,5 to 2,5 hours. For sight-seeing you can also book guided tours with jeeps. There are different kind of tours and you can join other people or book a private tour. The CKS also offers boat and jet ski tours. I've tried some of them. Not cheap, but worth the money.