Flash an LED with an extremely low power source
What you are asking for is not "relatively simple".

The biggest problem is the very low voltage.  You can connect a capacitor to the solar cell, then do something when it gets to a particular voltage.  The "do something" part would be possible just from the energy stored in the cap.

The problem with this is that something still has to decide when there is enough energy in the cap (its voltage is high enough) to use the slug of energy.  That something will take some quiescent current.  That quiescent current obviously needs to be less than the 2 &micro;A coming from the solar cell, else the cap would never charge.

Well under 2 &micro;A quiescent current is certainly doable, but not at a casual hobby level.  However, the real gotcha is that at 200 mV, ordinary semiconductors don't conduct at all.  It is just too low a voltage to operate from directly.

It is possible to boost a voltage, but that takes control current too, and will also loose some power.  If a higher voltage were available, a very carefully designed circuit could possible make use of 200 mV, but the allowed quiescent current goes inversely proportional to the voltage you boost too.

The only way I see this working out is if you get multiple solar cells and connect them in series to get a higher voltage.  I would want at least 1 V.  2 V will be much easier.