I am searching for ways in casual (informal, everyday) language to express elegantly that something which initially seemed (or could have seemed) a difficult task working out surprisingly easy once you started doing it.
Referring to all dictionaries in my reach I so far found only
Bob's your uncle
So, if this is true, I could say
We worked on that problem and were prepared to spend several hours on it, but - Bob's your uncle - a solution came up.
for what in German might be expressed by
Wir beschäftigten uns damit und waren darauf eingestellt, dass das Stunden dauern würde, aber dann, schwuppdiwupp, fand sich eine Lösung fast von selbst.
However, I am not sure if Bob's your uncle is really appropriate, or if it should be used rather in a family situation, with children etc. but not so well in ordinary (say: workplace) situations. It was simply the only thing I found for schwuppdiwupp so far.
PS: I might add that schwuppdiwupp in German is an onomatopoetic expression depicting some objects (timber, or a piece of soap, or whatever) that move surprisingly quickly and without apparent effort.