I've never been to Huelva, and if seems to be an unusual cruise destination.
Most people bypass it on the fast bus from Portugal to Seville. I have always planned to visit Huelva on the old slow rail and ferry route from Portugal to Seville, not knowing how interesting a place it seems to be.
There is a Visit Huelva website, and it sounds a really interesting city, chock full of History.
It seems to be what Carlos is often saying here, that there are lots of other interesting towns and cities in Spain off the usual tourist trail which are well worth visiting.
Apparently the cruise dock is in the city centre. If you look at the City website I'm sure you could put a self guided walking trail together. I know I could. But it looks the kind of place which would also reward richly a bit of wandering.
Not being a regular cruise port I would imagine there will be an information desk set up on the quayside.
There may well be local tours waiting on the quayside.
Maybe also get inspiration from the descriptions of whatever local tours the ship is offering.
To me this is when cruising gets interesting, discovering lesser known places you wouldn't think of going to.