How do I make a normal looking cave without TNT?

Another option (which I have experience with) would be to use the /sphere command in WorldEdit. I have created some descent looking caves by making many holes of various sizes underground.

Try voxelsniper instead, you get some expirence, and you don't have to do all that typing or /powertool sphere on anything

But Blargh I don't know voxelsniper!

I don't know if you can add yourself but /addsniper (yourname) if it doesn't work get another sniper to do it. For this specific function, after being added to the sniper list, you use /v 0 (v is for voxel-as in block) and /b (radius of brush) you then do /b b for the ball tool, and right click away with an arrow.

ok, now its a very man-made looking tunnel, and I messed up a couple times

Use /u to undo, and to make it look more natural you can use the erosion brush, with /b e smooth it smooths the place out a bit, and with /b e melt you can burn through the terrain a bit.

Anyway thats just a basic way for tunnels with voxelsniper, even if it was an unwanted explination.

Also say /d if you ever want to go back to the defaults