What visa should I apply for to stay 91 days in Thailand, or should I do a border run after 30 days?

To stay in Thailand for 91 days, you could apply for a visa that allows for a longer stay, such as the NON-O visa. Alternatively, you could enter Thailand on a 30-day visa exemption and then extend your stay by 30 days at immigration for 1,900 baht. Another option is to do a border run to a neighboring country like Cambodia (Phnom Penh) after your initial 30 days, which could reset your visa allowance.