We were lucky that DS still fit in a bassinet when we flew to Thailand at 20 months old. However, we only used it on the night flight. On the day flight over, he was too excited by all the activity and was pretty happy but required constant entertaining. If you have a day flight, try packing lots of activities - sticker books, interactive toys (don't worry how noisy they are lol! The noise of the plane drowns out annoying toy sounds), plenty of snacks and an iPad loaded with Peppa pig/whatever shows they're into. Lots of walks up and down the aisle. Still try to get a seat at the front of a section (ie where the bassinets often go) as you will have more room to get in/out and be close to toilets etc. even if you are not successful ask at check in and again at the gate as we often find a bassinet seat (or similar) suddenly becomes available. The in-flight staff will most often try to accommodate you as best they can.
If all else fails, have yourself a wine and do the best you can, knowing it will all be over in X hours! Good luck!