"Vague memories of chemistry lessons, but was it the Na that fizzed and floated on a cushion of hydrogen while the Li did the same but produced enough heat to ignite the hydrogen?"
No, it was potassium that did that. As noted by someone above, lithium is less reactive than sodium. Potassium is the next one down the list and significantly more reactive. You might also have seen video of rubidium, which is even more reactive to the point they don't generally allow it in schools. When dropped in water, lithium gives a dissapointing fizz, sodium floats around on a cushion of hydrogen, potassium ignities the hydrogen (usually in a series of pops rather than a constant flame), rubidium blows up the water tank.