Have a wander around the Albert Dock definitely and along the pier head to the three Graces and the canal.
walk back up through Liverpool one, the big shopping area and then head down Bold Street
head up the hill to the Bombed out church (literally the remains of a church that was bombed in the war but very pretty and interesting). On your way up if you fancy a drink Arts Bar in the Masonic hall on Hope street is fab with love artists of all kinds on.
go for a walk around Paddy’s wigwam, the “new” Catholic cathedral, really interesting as it’s in the round. The catacombs are interesting too.
then walk across (and up the hill) to the other cathedral and you can climb to the top for amazing views of Liverpool.
on your way back down, go down Duke Street, stop in Duke Street Market if food.
have a walk up to the World Museum, z Walker Art Gallery and St George’s Hall, the latter has a really good small museum about when it was used as a court house - hidden gem!
head back into town and go to the cavern club etc around Matthew street.
then head towards the Baltic triangle, lots of cool and quirky places to eat plus an amazing ghetto golf for evening entertainment
i think someone’s mentioned up thread but the Williamson tunnels are also a hidden gem