Hi Samir, In my opinion, both the bare infinitive and the infinitive are correct depending on the situation. Let me explain to you why. My examples: “have to” structure All I had to do was THAT I HAD TO watch television. All I had to do was TO WATCH television. All I had to do was WATCH television. All I had to do was WATCHING television. All four sentences are grammatically correct. The first one uses a “real noun clause”. The second one uses a “to + infinitive” form. The third one uses an “elliptical noun clause”. (The same as the first sentence, but it is reduced.) The fourth one uses a gerund. All of them are acting as nouns. Your sentence: All I have done is THAT I HAVE WATCHED television. All I have done is TO WATCH television. All I have done is WATCH television. All I have done is WATCHING television. Here, this is similar to my examples. However, as you can see, a “real noun clause” in your sentence should use the “past participle” form of a verb, “watched”. “I have watched.”, not “I have watch.”. This is because your sentence is not a “have to” structure, which is used for obligation. Knowing that, I would say that the third sentence, your sentence, is grammatically wrong. The first, the second and the fourth sentence are grammatically correct in your sentence. Please note that in a everyday conversation, it is acceptable and understood when the third sentence, your sentence, is used. This is just colloquial. I couldn’t give you any link because this is just what I think about it based on my knowledge. If you are wondering about the “elliptical noun clause”, here are both an elliptical adjective and an elliptical adverb clause: Adjective one: Describing Talking to Zoe, Tim is happy. Elliptical clause. Tim, WHO IS talking to Zoe, is happy. Complete clause. Adverb one: Showing relationship (WHILE) Talking to Zoe, Tim is happy. Elliptical clause. Tim is happy BECAUSE HE IS talking to Zoe. Complete clause. Does that make sense?

https://www.ef.com/english-resources/english-grammar/present-perfect-vs-simple-past/

Thank you, Melanie. Btw, for native AmE speakers it also sounds correct and natural.

As a native speaker of British English, I find the original sentence 'All I have done today is watch television' correct and by far the most natural. Similarly: 'what would you like to do today - watch television or play tennis?', and 'I was so tired, all I wanted to do was watch television/sleep/go to bed, etc'