Are we talking about high current LEDs? or just regular 10-20mA? If they are lower power, a low value potentiometer wired in series will limit the maximum current (and hence brightness).
You want the Pot resistance to be at the normal setting near the middle of it's range (e.g. use a 200 Ohm pot if you will typically use 100 ohms) and make sure the power rating of the pot is 2 or more times what the expected power is (pots can't dissipate full power near the low end of travel).
If they are high power LEDs you probably want a current limit that is controlled by the pot.
Steve