Invention motives are generally short and melodically simple (which is not to say simplistic!). At the musical surface, many consist of straightforward arpeggiation or scalar patterns, or a combination of both. An analytical reduction often shows that, just beyond that surface, invention motives are constructed from subsurface, rudimentary arpeggiations or stepwise note patterns.
Look at the motive for Inventio 1 (m 1, beats 1-2 plus the next 8th note). Assuming C-major harmony for all of m. 1, reducing away the passing and neighbor notes of the motive reveals a subsurface arpeggiation of a C-major triad in beats 1-2 plus the next 8th.
Look now at the motive for Inventio 4 (mm. 1-2). Assuming the harmonic progression in mm. 1-2 is (in D minor) i - V (or perhaps i - vii7), analytical simplification (removing passing and neighbor tones) reveals that the subsurface is again arpeggiations, first of the tonic and then a dominant chord (V or vii7).
The same holds for Inventio 3. Mm. 1-3 are a surface elaboration (read: diminution) of an underlying, subsurface D-major triad. Likewise for Inventio 7. The motive (m. 1, beats 1-3) is a passing/neighbor-note diminution of an E-minor triad. Some invention motives consists exclusively (or almost) of arpeggation (Inventio 8, 10, 13, 14).
In sum, invention motives are melodically fairly rudimentary. This is in keeping with the very notion of an "invention" which, as already pointed out, is simply a basic idea, a kernel, from which a piece is built. The simpler it is melodically (and rhythmically), the more opportunity there is for varying and expanding on the kernel (i.e. developing it).
Inventio 4 provides a good initial example. We see the countermotive in mm. 3-4, played against the left-hand statement of the motive. Note that the countermotive is slower rhythmically than the motive, thus allowing each to stand out against the other. Further, the countermotive is rhythmically straightforward, i.e. not complex (all 8ths, no dotted rhythms). Finally, it is directionally opposite to the motive in m. 4, giving each voice a distintive melodic contour.
Examine Inventio 5. The two voices contrast in similar ways: slower versus faster rhythmic figures, and ascending versus descending contours. We see the same kinds of contrasts between motive and countermotive in Inventio 6, 8, 12, and in others.
In sum, then, contrast in rhythmic activity and melodic contour is a hallmark of the relationship between motive and countermotive in inventions.