It's amazing how many differences a small change can make. I started messing around with mapgen options in the config file because there's too many caves for my liking. On some maps there's hardly anywhere you can dig without coming across them. In fact on some maps there's hardly anywhere to dig. So I'd change one parameter at a time then play to see the effect. With my latest effort there's now quite a bit of beach sand in many of the caves, I've even got a lava pool surrounded by sand. And yet there's only one small change from the default. Here's the line I added to my config:
mgv6_np_cave = 6, 6, (150, 150, 150), 34329, 3, 0.50
The only change is the numbers in the brackets, I changed them fron 250 to 150. Lava with sand - try map seed 7369 12244 07196 57414 (spaces are only to make the number easier to read, don't include them) location 574, -36.5, -54
Cheers,
Andrew.