You can either use a mod or use a command to enable fly like in creative. Check under Fly/Speed in the donation features here.

 

Fly is pretty amazing and well worth it.

Fun things to do:
Never walk again. lol
Check out all the towns. (some have amazing pixel art, some are inaccessible due to height/depth or blocked portal)
Easily Build skyscrapers.
Dig straight down (while flying so you don't fall in lava)
Traverse underground caverns. (easy to mine ores that are halfway up walls etc)
Underground speed (easy to get around caves while looking for surface ores)
Create new mines in the wild (most fun I've had, fly out to a spot in the ocean, dig down 3-4 and plug the hole above you, dig down until you find a cave/bedrock/lava, explore to find untouched/unfound ores just sitting on the surface everywhere).

RagMan

 

Plus you move much faster so you get where you need to go.
Flying in the aether is handy (I have aether and it's awesome)
Plus come on. Flying is A lot of fun. Lol

 

Get it!

 

what mod do you need to get?

 

I suggest CJB modpack.