The Asus Zenbook Prime (13") is a very good Ultrabook at the top of his budget, while the Vizio 15" Thin+Light is a good choice for around $600 (if you buy from the Sandy Bridge generation). They are both very portable, with solid keyboards and 1080p IPS screens. They should also both have an SSD.
The Zenbook is a bit lighter and thinner, and has user upgradeable RAM (up to 10GB; one slot is a soldered in 2GB and other slot is user replaceable).