Kinokuniya is HUGE and probably has the widest range of books and stationery in Sydney; that, and Abbey’s Bookshop and are less than 10 minutes by bus from Sydney Uni.
Gleebooks is basically a walk across the road from uni.
Better Read Than Dead and Elizabeth’s Bookshop) are both in Newtown; about a 15 minute walk from the uni if you don’t want to bus it.
My vote for your gift vouchers would be Kinokuniya and, (with thanks to Askance’s suggestions) Better Read Than Dead.
Just thought it useful to know, abderian because if your friend loves browsing the bookshops rather than just ordering online, they’re all really close to where he’ll be expanding his brain.