The Tarntawan Place hotel advertises that it has wireless Internet in the rooms, and for no extra cost.
http://www.tarntawan.com
The SalaDang Place serviced apartments on Saladang Road say they have wireless Internet in the rooms, with various daily or weekly packages available. Ask them for particulars by e-mail.
http://www.saladangplace.com However, I understand they don't allow visitors without charging a joiner fee.