Day trip to Tangier from South Spain. Recommendations?

Forget Tangier, it is NOT a great visit for Morocco.
If you go to Morocco visit Fes, Marrakesh, Rabat or Agadir. Tangier is a waste of time.

We did this in 2014, stayed in Tarifa, took the ferry over, met a guide (I believe a RS rec, check his guidebooks). Weather was predicted to get rough later in the day, so we got a very rushed tour of the city (it was a Friday, and many things were closed after midday prayers) and the caves of Hercules (eh) because we would have to leave earlier than planned. Our guide ended up driving us to the commercial port to take a larger ferry, which landed in Algeciras. Which is not Tarifa, obvs. Only help we got from our guide was "There will be free shuttles" - but the ferry office wasn't running them because the ferries were cancelled (I am not making this up, honest). Wandered around, finally found the bus station and a bus back to Tarifa. Now, this was 2014 - so no or limited international cell plan (probably relied on free wifi), which would have helped to solve several of the logistical issues. BUT - would I do this again, even without all the issues? Big maybe (and now I know what to ask, like what's the Plan B for the ferry; is everything open that we want to see; can we skip the sales pitches for carpets; etc). Lots of other things to do in Southern Spain, depending on your travel plan.

Personally, I wouldn't bother. But if you do decide to go ahead, check where in Morocco the ferry arrives. From memory, the Tarifa ferry arrives in Tangier Ville (town), near the centre. But the Algeciras ferry arrives at Tangier Med which is about 30km from Tangier itself. Algeciras ferries also go to Spain's Morocco colony in Ceuta, which wouldn't work for a day-trip to Tangier.

Morocco has many wonderful destinations. Tangier is not one of them. There are flights to places like Fes and Marrakech from many cities in Europe. My recommendation is that you not spend time on a trip to Spain detouring to a coastal port you would otherwise not visit and taking a ferry to a disappointing place like Tangier. Instead, plan to fly to Fes or Marrakech (and see both, and maybe other places as well) on a later trip to Europe. You don't have to settle for Tangier just because you will be in Spain this year.

Ask anyone who tells you "Tangier is awful" how long ago it was they went there - or if indeed they've ever been there or are just parroting legends that they've heard from someone who knows someone. Tangier USED TO BE quite seedy (although that in itself had its own charm), but the new king and government have invested heavily in it (and European tourism in general). If you only have a day to see Morocco, Tangier is your obvious choice.

My concern with your plan is not the Tangier part, its the day-trip part. The Algeciras-Tangier Med option is absolutely no good for a day trip (although we use it because we stay in Chefchaouen, closer to Tangier Med than to Tangier). The Algeciras-Tangier Med ferries take 90-120 minutes, and Tangier Med is 30 minutes from Tangier (plus processing time in the port).

Your ONLY choice for a day trip is the Tarifa-Tangier Ville ferry, which takes an hour and drops you right in central Tangier. Problem then is getting to Tarifa from wherever your day is going to start. If you have a car, and are close enough to be able to get to Tarifa early in the morning, that would be best. There are buses to Tarifa, but the bus station is not near the port (except the ferry company FRS runs a shuttle from Algeciras Port to Tarifa Port (if getting to Algeciras Port is easier for you than getting to Tarifa Port).

All in all, its an awful lot of logistics for a day trip, if you ask me.