Check in a hotel for one night. But first ask them if they file TM30. (tomo samsip in Thai) Tell them it's for your visa. You have to make sure they will do it and ask for a copy because not all hostels/hotels do that automatically.
If they say no, move to another hotel that does. Request for a copy.
You can also use bangkok bank or other thai online banking which i use for 711 delivery or cash. These two I use. Better to use cash because more often than not there are items from your order that they dont have at the time of order and cash makes it easier to pay the exact amount in case changes occur.
i dont know what your visa is, it depends. You can either wait in Laos or in Thailand: If you can extend your visa at the immigration, good. If not you can exit thailand first and enter Laos on visa exempt (that depends on your passport) And then enter thailand on visa exempt also (again, it depends on your passport) that will give you 60 days. Or just stay in Laos while waiting for your online appointment.
James Miller is right. No online appointment, no entry. Only those with online appointment for the day are allowed to enter the embassy. They have a list of names in their book. If the name is not there, they dont let the person in. We were just there few weeks ago. Also, the guard checks if the person with appointment has the complete requirments, if not complete, he sends him out to complete it first.
You book hotel again meaning today or asap. Thwn ask the hotel staff to register the TM30 for you for immigration purposes. Ask them for a copy of the TM30 registration. Tell them you are going to the immigration after you boom the hotel. You have to tell them because not all hotels register the TM 30 if you dont tell them that you need it. So better, when you go to the hotel, ask them you need TM30 registration. If they say they dont do it, then m9ve to another hotel that does. It is their responsibility, but many small guesthouses dont do it. So better go to a hotel that does.
It will be faster if you print and fill out the re entry form (can be downloaded from thai immigration website) and bring your own photo . Than let them do it for you. Especially if there are many people in line for the re entry.
You will save a few hundred baht and it will only take a few minutes when you can hand them everything they need right away than fill it out there and have your photo taken there. This is good especially if you are running out of time.
But I recommend you really go to the airport early.
It wil be helpful and faster if you print the extension form (you can download from thai immigration website) and fill it out and have your photos ready before you go to the immigration of the city where you stay. Also, you can book one night in a hotel and request them to do the TM30 for you and bring that TM30 copy/hotel receipt with you when you do the extension. The imm. officer will ask for that.