Example of an achiral molecule without a plane of symmetry or inversion center?
Part 1

Are there any molecules lacking plane of symmetry/center of inversion but that are achiral due to presence of an axis of improper rotation?

Yes, although such examples are very rare. In general, the point groups $S_{2n}$ $(n \geq 2)$ are where you should look. These molecules have a $S_{2n}$ rotation axis, but no plane of symmetry, and no inversion centre.
Molecules in the $S_6$ and $S_8$ point group are are exceedingly rare, and generally need to be constructed specifically to satisfy these criteria. The best examples, therefore, come from the $S_4$ point group. You can find several examples on these websites:

http://gernot-katzers-spice-pages.com/character_tables/S4.html
http://csi.chemie.tu-darmstadt.de/ak/immel/tutorials/symmetry/index7.html#Sn

In particular, one of the nicest examples is 2,3,7,8-tetramethyl-spiro[4.4]nonane from the second link. In the graphic below, I've taken the liberty of using four different coloured atoms in place of the methyl groups. The reader should therefore bear in mind that the third image is the same thing as the first, even though the colours are different.

The XYZ coordinates that I used for the pictures above are as follows. You can download these and view them in software such as Avogadro or ChemCraft, although I don't think you can perform a &quot;reflection&quot; per se (I manually swapped the colours between the second and third pictures to achieve this effect).
C         -2.32545        1.12231        0.91714
C         -2.19292        2.47894        0.24035
C         -0.83350        2.34003       -0.44571
C         -1.75438        0.10430       -0.11277
H         -3.31782        0.88874        1.35769
H         -1.62382        1.17900        1.77968
C         -0.98281        0.99078       -1.13371
H         -1.65567        1.19844       -1.99603
H         -0.05915        0.56089       -1.57585
C         -2.87426       -0.73718       -0.79187
C         -0.83536       -0.95866        0.55714
C         -2.33811       -2.16138       -0.80738
H         -3.27791       -0.35456       -1.75318
H         -3.74616       -0.78244       -0.10108
C         -1.65652       -2.23999        0.55910
H         -0.36215       -0.67792        1.52187
H          0.00882       -1.17785       -0.13479
B         -1.40058       -2.29939       -1.82101
H         -3.13035       -2.92594       -0.95624
O         -2.60331       -2.18962        1.57227
H         -1.04117       -3.15429        0.69961
H         -2.24954        3.33056        0.95160
F         -3.19594        2.63169       -0.70627
N          0.17857        2.27493        0.50135
H         -0.60168        3.16745       -1.14998


Part 2

Are there are also examples of vice-versa, where a molecule has a plane of symmetry/center of inversion, but lacks an axis of improper rotation and is thus chiral?

An improper rotation $S_n$ is defined by a rotation about $360/n$ degrees, followed by reflection in a plane that is perpendicular to that rotation axis.
A plane of symmetry ($S_1$) and an inversion centre ($S_2$) are special cases of an improper rotation ($S_n$). It is easier to convince yourself of the $S_1$ case: according to the definition above, $S_1$ means rotation through $360^\circ$ followed by reflection in a plane. Since rotation through $360^\circ$ obviously does nothing, this is the same as a reflection in a plane.
So, the answer to this question is no. if a compound has either a plane of symmetry or an inversion centre, that automatically means it has an improper rotation axis.