One of my bikes, a Trek Pure, is super fun to ride, but it's not fast enough. I max out on the flats (not to mention downhill).

It's a seven-speed with a 42t chainring in the front (one ring only) and a seven-speed cassette in the back that starts at 14t.

The upper end is fine. I don't have to climb excessively steep hills and the gearing in that area doesn't need anything.

I simply want to go faster on the flats.

Should I get a larger chainring or a smaller sprocket on my cassette? And which would be easier (and cheaper) for me to do?

I don't want to make a big deal of it as I'm purchasing a mountain bike soon for a real workout. In the meantime I just want to be able to go faster than this beach cruiser will let me.

Oh, and I don't care about the bash guard.

Thanks.