What are the options for extending my stay in Thailand until April next year with a tourist visa?

If you are in Chiang Mai also consider doing a border run to Mae Sai (Myanmar border). This is quicker and cheaper than flying to Udon and going to Vientiane- it is 4h40 on a comfortable bus direct from CM, so you can do the whole thing there and back in one day.

You can only get two visa-exempt land entries per calendar year, but this would reset Jan 1 so if you don't have any yet, doing one in December would not take anything away from your quota.

You get 30 days doing this (rather than the 60 you would get with a visa) and you can extend by 30 for 60 total (rather than 90 with a visa+extension).

If you do a run on Dec 6 you would be stamped until Jan 4, with extension until Feb 3.

You could then at that point either do a visa run to Vientiane for a further three months, with extension, which would bring you through to the start of May, or alternately do another bounce at Mae Sai which would, with extension, get you up until Apr 3.

If you need until end of April, 1 Tourist Visa, extended + 1 border bounce, extended, would get you there. If you are doing that combination, it is better to do the border bounce first, in December, as that will reset on Jan 1 and doesn't "cost" you anything from your quota.

Check the date stamped when you apply for your Extension of Stay, before midnight that day you have to leave Thailand. You can apply for a new Single Entry Tourist Visa at the Thai Consulate in Vientiane, apply day 1, pick up next working day. For other advises and travels I suggest you use the search button as they way to get to this Thai Consulate are many times mentioned.

Just be aware of your stamped date admitted until when you make a schedule. Enter date is day 1 and October has 31st, that means the last day cannot be 8 Dec...

Flying to Udon Thani and getting to the border (Nong Khai) from the airport is super easy. Cheap minibuses directly to the border, and takes around an hour! I really recommend that way over buses overland!