I'm going to be visiting Bangkok for one week, and would like to have a contingency plan in case the hotel WiFi is unusable.
What are my options for a data only SIM card for one week?
All three of the major operators sell prepaid SIM cards with a data allowance from their counters on level 2 or Suvarnabhumi airport. Providing you have an unlocked GSM phone / tablet (no CDMA), you can have 1GB of data for 7 days for 300 Baht (less than US$10).
True and DTAC both have 4G networks, and the tourist plans also have free wifi which is widely available in almost all public venues. Getting set up on the wifi involves sending an SMS to get a password (the username is your phone number). You can ask the staff when you buy the card and by should be happy to set you up.
There aren't so many data only / tablet / midi SIM options, and asking for one may complicate the issue. My advice would be to go for the tourist smartphone sims, have the staff set you up before you leave the counter.
Topping up is simple and can be done online or in any 7-11 store.