Welcome to the forum!
I'd suggest that you start shooting pics first. That will help you to decide which types of shot you like best. Shortly after I got my camera, I turned up at the coast to take some landscape shots, but ended up taking photos of wild flowers in the rocks.
Very wide angles are good for landscape, such as the Nikon AF-S 12-24mm f/4G IF-ED DX. Consider protecting your lenses with a Hoya HD protector filter (not UV). You will also need a method to clean the lens such as a Lenspen, also a Rocket Blower by Giottos. I have a Blackrapid wrist strap myself, which works well. Manfrotto make the very best tripods, but decide which types of shots you will want to be taking first. Tripods are good for staged family portraits, landscapes & macro/close-ups. Bear in mind that for a staged portrait, you will need good flash lighting.
You might like to get a second Sandisk memory card, but no need for a 2nd battery unless you'll be going on long weekend hikes. Hang on with the backpack until you've figured out exactly what type of photos you want to take & therefore know which lenses will need to fit in the backpack. Good makes are Lowepro & Thinktank. Get a cheap should bag for now. I bought a Thinktank 5 retrospective as a go-to grab bag.
Indoor & outdoor candid shots would be good with a fast lens such as the already suggested Nikon AF-S 35mm f/1.8G DX.
Consider whether you want to shoot in RAW or JPG format & if you choose the higher quality RAW, make sure that you have the computer spec & software to handle it.
Best step at the moment is to get out there with your current lens & do some shots.