Quick answer
Card for almost everything, cash for the small stuff. Carry a travel card with clear overseas terms for contactless and ATM withdrawals, and keep 2,000–3,000 THB on hand for street food, tuk-tuks, temples and the shops that still do not take cards. Thailand is increasingly card-friendly, but it is not yet cashless once you leave the malls.
Planning a longer stay? If you are moving beyond a holiday, check our Thailand cost of living guides for monthly budgets by city, the UK retiree money checklist, and health insurance for expats.