The Old Gaol is probably the number 1 single item of interest.
NantClwyd house - old.
The square, including the 'Seven Eyes' aka 'Myddleton Arms', with it's famous windows. Apparently it's called 'The eyes of Ruthin', except by anyone who's ever been there.
Ruthin Craft Centre - this was allegedly going to become a petrol station for Tesco, but instead, they knocked down the old grey shed-like building, and replaced it with an expensive new grey shed-like building. Apparently, the architect intended the roof to emulate the pattern of the Clwydian range that lies behind it. However, Moel Famau looks nothing like an aircraft hangar.
A really expensive work of art can be found hidden away near the council offices for their own personal benefit at taxpayers' expense.
Ruthin Castle is private grounds, though you can probably get a few drinks at the bar and then walk around.
Some out-of date food by that place near Cae-ddol. Gaudy confectionary at sweetie heaven. A surfeit of hairdressers. A couple of clothes shops. Some newsagents, ironmongers, carpet stores, greengrocers, stationers and toy shops.