There are many interesting ways to say this without mentioning the word toilet. Bathroom, men's room, ladies' and so on. I've also heard washroom being used (interestingly in Kenya or by Kenyans). I don't think Duo should accept all these euphemisms, but it would be interesting to know if there are any common ones in Swahili.
Yes, 'ninaweza kujisaidia wapi?' is the most common term I have heard... Toilet is not strictly correct: it seems to be a translation from the French - toilette - which actually means to wash oneself, to perform personal hygiene - which is hardly the same as using the lavatory!