Smallest, simplest possible (randomly) flashing 2-3 LED circuit
lets start with Specs. and assumptions;

CR2032 Nominal Voltage: 3.0 Volts
Typical Capacity: 240 mAh (to 2.0 volts)
(Rated at 15KΩ at 21°C)
Battery ESR ~ 3kΩ

Red &amp; Yellow LED = 1mA@1.9V , 20mA@ 2.2V +/-10% Voltage 
thus from 1~20mA, ESR= 0.3V/20mA = 15Ω

let Flash time duration = 100ms   RC=0.1 , R=15Ω ∴C = 0.1/15 =6,600 μF,
 ∴ consider lower duration as this will cause problems for charge time

Resistance ratio = R_bat/R_led = 3,000Ω/15Ω = 200 which means charge time is 20 seconds and thus either one needs to reduce battery reistance ( bigger battery ) or raise LED resistance ( lower current from 20mA to 2mA ) or reduce flash duration (preferred).

(Take 2)
Let Flash duration = 10ms, C= 660 μF and consider raising LED current to 30mA

Compute Charge time from discharge Vmin to Vmax voltage:

Vmin = LED threshold , e.g. 1.6V for Red,Yellow
Vmax = 3V max, or less depends on charge time, eg RC rise time, ∆V=63% 

T = R_bat*C = 3kΩ * 660 μF =2 seconds.  where T is defined as ∆V=63%
thus after T charge time Vcap= where  3-1.6V = 1.4V  ∆V=63%*1.4=0.9V
∴Vcap=0.9+1.6=2.5V 

Simple solution might be a 5Hz CMOS gate relaxation oscillator (Schmitt inverter with 10MΩ feedback R, 1uF cap to gnd) driving a MOSFET in series with LED on Drain to  660uF cap which is in parallel with CR2032 battery

You can make your own Parts List and schematic for Q, two C's, one R1, one LED, one battery.

Random ? go digital.

I wanted to show the analog design process, not the solution.
A digital solution can be done by others.