Euros accepted in Basel?

Euros might get accepted, but only at an unfavorable rate, and with change given in Swiss francs anyway.
However, you do not need much cash in Switzerland these days. For 2 nights in a city like Basel, you could even get by without any cash at all - card acceptance is very, very good; better than in neighboring Germany. If that worries you, withdraw 20 or 40 francs at an ATM, but you probably won't need more. In early 2020, the only places where I needed cash in Switzerland were mountain huts (poor/no phone service usually = no card), farm stands / market stalls, and the odd locker. None of which you'll likely need in Basel.

When I spent a couple of days in Basel I used contactless payment for everything other than a Christmas market where I withdrew some Francs from a nearby ATM. I had Euros on me as I was going on to Germany however the exchange rate made it preferable to take out a few Francs.

You may need cash for some of the public toilets, especially at the train stations.

When I was in Switzerland this past June, the toilets at the Lucerne train station were in the process of being converted from cash only to credit card only. I do not know if this was being done at other train stations as well. The toilets at the Basel train station are pay toilets.

Switzerland was the first place I saw highly organized and efficient pay toilet operations in the train stations. Since then I have seen the same in Paris, well not quite as well run. But people were pretty surprised when they walked up and found that they had to pay at a turnstile so this seems to be a new thing. I had coins but it was possible to use a credit card. A few of the younger ones just jumped over when the attendant wasn't looking.

For two nights, as noted to be safe get 20 or 40 CH francs. Be sure to spend them before you leave, they don't work anywhere else. but you could always tip with them ...