Day trip to Tangiers?

Most of us would recommend not doing it. There's a lot of transportation overhead involved from any of the commonly-visited Spanish cities, and Tangier is not a particularly nice Moroccan destination. There are flights to better places in Morocco, like Fes and Marrakech, from many cities in Europe, so it is easy to include some time in Morocco in another trip to Europe. You don't need to squeeze a day in mediocre Tangier into this trip just because you'll be in Spain.

How many nights are you spending in Seville, Cordoba and Granada? Are you going to Cadiz and Malaga? There's more to be seen in Andalucia than most American visitors have time for on any given trip, so I hate to see precious time spent going to Tangier.

Tangiers, NOT

My recommendation would be stay in Spain and enjoy the wealth of what it has to offer. I did not enjoy my visit to Tangiers. It's a dirty, run down place apart from the district where the wealthy live. The food was mediocre, there is lots of animal abuse on display and hassle from people wanting your money everywhere you go.

If you want to go just to be able to tell everyone back home that you went to Africa (which is the reason most people seem to go) then do so if you wish but if you want to sample a taste of true Morocco then make the effort to go to the better cities.

In light of recent events you may want to avoid the area entirely.