D&D 3.x Running 3e D&D for the first time in a while, any advice?

[Celebrim]

Unless you have a high system mastery player trying to break the game, 3.X runs very well from level one to about level 7 to 9 if you stick largely to core and very sparingly introduce any other CharGen options.

If you want to run it at higher levels successfully a very large variety of balancing changes are required, too many to list here. But expect the game to increasingly involve a degree of negotiation between you and the players as to what is fair and reasonable as you level up.

 

[Voadam]

Oh, and I am planning to run a modified version of Expedition to Undermountain.

This is one I don't have. Undermountain is a big megadungeon though and I think this is a later 3.5 so the combat encounter stats should be pretty much in a page by themself, just be sure to double check the monster feats, powers, and spells beforehand so you don't end up missing out on relevant stuff as I was constantly doing with d20 monsters.

What levels are you running at?

For higher levels I suggest the 3.5 Unearthed Arcana optional rule of fractional saves and BAB so that the already big save disparity of single classed characters does not get more extreme for multiclassed ones (so a multiclassed barbarian fighter has the same saves as a single classed fighter or barbarian instead of tougher fortitude but weaker reflexes and will).

 

[Voadam]

Remember that 3.0, 3.5, and Pathfinder 1e are all very similar but each has tons of small different fiddly differences throughout their rule sets so what triggers an attack of opportunity or the specifics of spells will be slightly different between each one and it pays to look things up for the specifics as they are relevant even though you know the basics of everything.

 

[Lord Zack]

We are starting at 1st level and I agree with your advice on fractional saves and BAB.