Must Salat al-Isha be the last act in the day or can do something afterwards like watching TV or talking with family?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

It is not mandatory to make the 'Isha Prayer the last act of the day. In fact, discussing useful matters afterwards is permissible such as seeking beneficial knowledge, discussing issues concerning the Muslim Ummah, and talking to a guest. The evidence for this permissibility is the narration of Ibn ‘Umar who said: “The Prophet prayed one of the 'Isha prayer in his last days and after finishing it with Tasleem, he stood up and said: "Do you realize (the importance of) this night? Nobody present on the surface of the earth tonight would be living after the completion of one hundred years from this night.” [Al-Bukhari and Muslim]

Imam an-Nasaa’i entitled a chapter while mentioning the merits of ‘Umar by this title: “The Prophet used to talk to Abu Bakr at night about the issues concerning Muslims while I [i.e. ‘Umar] used to be with him.”

Abu Barzah reported that: “The Prophet used to dislike sleeping before the 'Isha Prayer and talking after it.” [Al-Bukhari]

The reason for disliking to talk after 'Isha is that it leads usually to staying awake for a long time at night which generally results in missing the Fajr Prayer or missing the Prayer at the last part of the night (Tahajjud) for the one who is habituated to performing it. Also, to avoid ending one's day talking in nonsense, etc. Therefore, the Prophet instructed Muslims to end their day with a good act; i.e., the 'Isha Prayer.

As for watching TV if there is any benefit in watching it, then you may watch it, otherwise it is forbidden.

Allah knows best.