I have just spent 3 hours in Doha and took the 777 bus to from the airport to the Pearl and back.

I enjoyed my self-guided "private" tour (I was the only person on the bus in both directions) which enabled me to see a lot of the city at a cost of 10 riyals, rather than however much it would have cost for a taxi. Since the bus never stopped, it would have taken just as long as a taxi which would not have been immune from afternoon peak traffic. OK, maybe the taxi would have sped on the expressway bit to the airport but that's just a few minutes.

I didn't have enough time to do anything else except take the bus and walk for 20 minutes around the Pearl (and actually it was rather too hot for that), but I suppose will go to the museum / souq next time, also by bus.

I did see this thread before my trip, but unfortunately the lady at the Mowsalat desk got really flustered* and I completely forgot to check whether card payments are accepted.

I'm not sure why it would be silly to withdraw 100 riyals to pay for a 10 riyal item, unless you absolutely never intend to visit Qatar again or you have some insider information that the Saudi/UAE blockade of Qatar and the resulting pressure on the offshore value of the QAR has prompted the Qatar Central Bank to abolish the use of cash.

*FWIW, I paid with a QR100 note and when the lady saw it, she became very flustered and muttered something (in English) which I couldn't catch, involving her computer not working - yet a social media website was loaded on it. She then wrote down the serial number of my ticket on her bookmark. It certainly wasn't the fact that she had to give me QR90 in change that made her flustered, because her cash box had about ten 50 riyal notes and twenty 10 riyal notes in it.