Question: I've got a silly situation. I have an Arduino board (2009). There's just one inbuilt LED but can't do much with it beyond blinking. I have five LEDs (3 yellow, 2 green), breadboards, jumper wires but not resistors. (And there is a total lockdown here) I wish to play with these LEDs but can't risk it putting it directly. I tried using them in series but that did not light them up.
I know each LED needs a 330ohm resistor but that I don't have. I even have a 2x 7 segment display too but the same problem.

Any way to use them?
Thanks.

Update:

On suggestions, we have got two resistors from a faulty solder gun (blue-grey-orange-gold = 68k) and a (brown-black-brown-gold = 100) and 1 IN4000 diode.

Answer: 
I'm sorry for you, but no, there's no way of connecting LED's to Arduino without resistors, without risk of damages to your Arduino's ports or the whole chip itself.

Even if you try to connect them in series of 2 (2.5V per LED) or 3 LED (1.6 v per LED), it is not advisable.

Don't you have a broken electronic device that you could scavenge for resistors? Even a burnt CFL or LED lamp can have some resistors that you could use. 330 ohm is just a **minimum** value, but as you want only an initial learning experience with Arduino, LED's values from 330 ohm to even 10k ohm can permit Arduino to light a LED safely.

CFL lamps have diodes, you could connect a series of 4 diodes to make a 2.4/2.6 V reductor, this series of diodes could permit using a lower value resistor (CFL lamps usually has a low value resistor, under 100 ohm).

What are the LED's colours? Their voltage depends on colour.
Anything between about 250 ohms and 10K will work fine. Surely you can sacrifice something and pull resistors out of it and maybe extend the leads a bit.