What can I do on an iPhone without service?

If I get my hands on a new smartphone, I want to know what I can do without service. I can't afford it. I need someone to explain to me what an unactivated SIM card is, what limitations I have without internet, and Wi-Fi. First of all, with Wi-Fi, do you have to pay for it like service on cell phones where you have free service for a certain amount of time, or do you have to pay for monthly?

I don't care about calling, texting, Facebook, Instagram, whatever. It would be nice to have them, but I just want to be able to play games and apps.

Use it as an iPod

Dude just buy an ipod touch instead has almost everything an iphone does plus same apps/games to download thats what I did cause I can't afford the iphone plan as well

Wi-Fi is free to use, other than what you already pay for internet. Like if your house has Wi-Fi. It doesn't cost extra to use the phone on it. You can actually do quite a bit with Wi-Fi. Almost anything except make a phone call. There's even apps that allow you to do that though too. I've had a couple unactivated phones and used them as Wi-Fi devices.

Get ipod touch or ipad if you don't want a phone calling plan, you can still use Wi-Fi and all apps and imessage.