A 'trip' may be self-organised, but depending on the context, 'going on a trip' is strongly suggestive of going on an organised outing. The first page of my google results for 'trips' is all organised tours and activities. If 'trip' is rejected for this reason, then surely 'excursion' ("a short journey or trip, especially one taken as a leisure activity") should also be rejected?
Тур and турне mean a journey, a trip somewhere and back home. The only time it means "tour" is when you are talking about a tour undertaken by a popular metal band, for example.
Экскурсия means a guided tour, i.e. a walk or a trip with a guide showing you things and telling you interesting facts and some history behind hese things.