What things can I do if I have someone's IP address?

If that IP is in same network with you then YoU can:

1.Get his sysinfo.

2.Find his OS details.

3.Scan for open ports.

4.Find whether that IP is using firewall or not.

5.Get the running services.

6.Can Perform Man-In-The-Middle (MITM) attack.

7.If the OS is windows XP,you can get the full access of the system without touching that system.

8.If the system is windows 7 +/Linux/Mac with some social Engineering skill you can get the complete access.

9.You can find the list of vulnerabilities of that device (low,medium,High)

10.You can use that system as BOT (after compromising the system).

11. you can go for DNS Poisioning attack.

12.Can perform phishing attacks too.

13.After getting access on Metasploit you can perform a lot of stuff like:

-reboot

-shutdown

-webcam chat

-webcam shot

-remote desktop

-sysinfo

-Hashfiles

-keyscan

-Microphone log

Rather then these you can control a lot of stuffs too.

14.Can find the versions details of Softwares.

These are Some common things which you can perform on your target IP.