Cambodian border would be OK, just avoid POI PET. The Myanmar border with Kanchanaburi I not sure if they stamp Mexican passport. If you fancy a trip by air you could frly to Chiang Rai and cross at Mae Sai. or fly to Udon and cross at Nong Khai. I will also quickly check entry requirements for Mexicans at Malaysia.
as a curious person I wonder why someone want to fly to Ubon, do a crossing to Laos (pay Laos arrival visa), stay a night there, afaik nothing there and return. There would be easier places to go.