Be aware that the kit lens you have will focus down to about 9 inches, so you can get some neat "macro" shots.
Don't know if the 3100 does this but the 5100 does. When you use the built in flash, it automatically runs the ISO through the roof resulting in very noisy pix which should not have been. I found the menu setting (you'll have to look; I don't remember where it is) that limits how far the camera can run up the ISO by itself and limited it to 800.
Keep in mind that you are not paying for film and developing, so experiment in manual modes and get a feel for what works for you.
See if the Nikon site has a downloadable pdf of the entire manual. As you look around here, you will find people referring others to a particular page in the manual only to have others not having that page because they've downloaded an abreviated copy of the manual.
Look at things from weird angles. Surprised a group of students when I lay flat on the ground in order to get a pic of the group and the entire dome of the capital in DC. Years ago, I was crawling under fences at Niagara Falls to get some dramatic shots. Parking garage roofs offer some interresting views. Shapes, colors, lines.
Oh, yeah! Don't forget about the video capability.