There’s a lot of information on the forum about the difference between nes i ddim and do’n i ddim but a quick response to these 2 examples is that gwneud is an action, something you were ‘doing’ in the past. With actions, we tend to use nes i or nes i ddim
but the second sentence has isio - “wanting” - it could be translated in clumsy English as “I wasn’t wanting to do too much last weekend” and it’s the isio that gives you the clue to the difference. “Wanting” isn’t an action, it’s a state of being, so that uses do’n i ddim