'Someone who hops from idea to idea'

Solution 1:

A dabbler might fit. Thesaurus dot com offers a bevy of synonyms, from which I'd pick a tinkerer. If solely intellectual effort is implied, a scatterbrain may be it. Turning to adjectives, fickle-minded sounds fine to me. P.S. Found another adjective: flaky

Russians tend to say that someone has "seven Fridays in a week".

Solution 2:

A dilettante.

Desultory fits if adjectives are acceptable.

Solution 3:

Two of the OED’s definitions of butterfly are: ‘a light-headed, inconstant person’ and ‘applied to persons whose periods of work or occupation of a place are transitory or seasonal.’ With careful handling, you might be able to use the word in the sort of context you have in mind.

Solution 4:

Scatter-brained, or simply scattered, come to mind: