For road use, those tires will be better than the stock knobbies.
Your fork has a mechanical "lock out". Use it unless you are going off of paved surfaces.
For tweaks, I'd start with a good quality saddle and ergo grips. Not to imply that the stock saddle is bad, but most folks find that this is an area of "customization" that provides a tangible benefit for their riding comfort. Expect to spend $60-100, and
measure your sit bone width to get an idea of the saddle width that will suit you. I can personally recommend ergonomic saddles (with the cutout in the middle), as they really do help to eliminate perineal pressure.
For grips, I like Ergon. The
GP3 would be a nice choice for that bike, as the integrated bar ends offer an alternate hand position.
The other thing in my opinion would be pedals. If you are content with platform pedals, then a set like
this will offer a nice platform with better quality than the stock pedals.
Next up in my opinion would be brakes. The stock brakes are good, but Avid BB7's with Speed Dial brake levers would make a noticeable improvement. I've used the Tektro Novella, and consider them to be equivalent to Avid BB5, which are okay, but not great.