I was pretty pumped to put this map together particularly the Abyssal variant which I think works well for a River Styx motif also. I think this map, while fairly straightforward, provides a lot of opportunities for unique encounters. The two main obstacles here are the river itself and the various elevation options. Whether it's river crossing or river traveling via boat it should work well. You can add baddies who reign down hellfire gauntlet style while the party has to balance attention between defending themselves and succeed a skill challenge to make it over the rapids!
[30x100] Grid and Gridless Versions