The short answer is: Everything.
When traffic leaves your house, it's in your ISP's domain, and there's no way of getting around that.
The long answer is more complicated. If you use encryption wherever possible, payloads would not be viable to view (it may be possible to decrypt a packet, with enough time and processing power, but it's not a viable use of resources).
What they can most definitely see is the information of where it's going. IP address, port number and below are considered to be public, for all intents and purposes. They can see where you went, and when, but not what you did.
The mitigating factor here is where you introduce an additional step between you and your destination. A VPN, for example, encrypts all traffic and passes it as is to the VPN endpoint. From there, it acts as if you were browsing from that endpoint. The ISP for the next step would see that someone visited Google, but you're account is separated from it.