So, I am going to start a new game, using a cool character concept: the Kensai Druid. The big thing about this class to me is barkskin. Since druids have kinda worthless 2nd level spells, I will fill the 2nd level with barkskins and get a kensai which can summon his own natural armor. As well cool druid spells, better melee capability, and so forth. The problem is, what kind of weapon will I be using. I have already decided against dual wielding, since then I'd be limited to only the club and scimitar. Can you guys give me advice, since I'm stuck... :(

Firstly, good luck rolling. The Kensai will need high tanking abilities (STR, DEX, CON), and the the druid abilities (WIS and CHA) need to be very high to allow the dual, which only really leaves INT as the expendable stat with a fighter/druid, and you'll probably want that to be above 9 so you can make use of protection scrolls and aren't treated as a gibbering idiot. Also, since you have to be a human for this to work, the 9/3/3/3/3/3 stat minimums are likely make rolling even harder.

I'd recommend aiming for:

15 STR (at least! 18 would be nice)
18 DEX (to maximise AC)
18 CON (to ensure that the first 9D10+CON HP bonus is max)
10 INT (for conversation and protection scrolls)
17 WIS (at least - the minimum this can be)
17 CHA (as above)

Which requires a mind boggling 95 ability roll, 98 if you want to start with 18 STR!! I know this isn't what you asked, but just beware of the extra-ordinairy stat requirements for this class!

In terms of weapons, it really depends on what patches and mods you have installed. If you aren't using the grandmastery patch, then my advice would be to couple Belm (offhand) with BackBlood, or even Gnasher, since high APR will make the kensai's bonuses shine. If you are using the GM patch, then (assuming you've reached grandmastery in a weapon of your choice) your APR will be exceptional by level 13 (possibily the best dualling point if you intend on playing through ToB too), so I'd recommend Backblood (offhand) combined with Gnasher (the "biting" sytle damage will quickly mount up, especially with a high APR rate). Alternatively, you could take an entirely different route and use spears, with Impailer being your first choice. It's really up to you, but there's plenty of great weapon combo's for a fighter -> druid.

if you're sure about not dual wielding ;) and assuming you dual at 13 how about this

quarterstaff ***
two-handed weapon style **
spear ***

no shields of course 'cos of the kensai

ooh - daggers will give you a kensai-friendly ranged weapon don't forget

Well, I'm going for a more moderate roll, more like: Str 15
Dex 16
con 10
int 10
wis 17
cha 17

That's quite do-able. And woa, had forgotten about the quarterstaff... There are some nice staffs available, hubba!

Let me also recommend you put as many points into quarterstaves as possible. I played a Kensai/Druid and relied on the Staff of Rynn for virtually all of SoA. Also, started her with 12 strength (to place more points in areas like Dex, Wis, and Cha) and then used strength enhancing gear to beef up her stats. Overall, this was a great character to play. So much so, that I'm probably going to solo one once I finished my solo Blade.

Staves all the way. We always forget about them, but the staff of rynn (that +4 one available at the adventurer's mart, I believe) is available early on. Then you've got a seriously powerful staff in TOB. Spears is also a good thought.

If you have the item upgrade mod there's a club that gives automatic grand mastery, so no need to put any points there... I had a certain TOB character dual wielding clubs one game just because he looked cool that way.