I think most humans like a bit of variety - so doing one thing for a long time probably won't keep you happy for all that long.
Exercise - 30 minutes of decent effort - gets you panting and sweating - releases endorphins. Endorphins are feel good drugs. So you might want to pick an exercise and do it for 30 minutes as hard as you can stand - when you DON'T FEEL LIKE IT.
In fact if I start doing something I want to do but don't "feel like doing", I often start to enjoy it after I get started.
Can't decide what to do first?
write a list.
sort it in to quick things that don't take long
and things that will take a while, like hours, days, weeks.
sort both lists so that the most important things are at the top.
Start with the quick list.
roll a 6 sixed dice - do the thing that corresponds with the number you roll (that's if you don't just do the number one thing).
Then reward yourself somehow. waste some time. whatever. read a scifi novel?
Then look at the hard/long list.
Start with the top thing, and see if you can't break it up into steps. break the steps into steps, until you have something that fits on the quick list.
Things I like doing but rarely feel like when I start: walking, bike riding, helping out as a volunteer - especially stuff that involves animals (join a club), scuba diving, visual art - either making something or visiting a gallery, reading, building websites (I do them for my clubs) etc.
Things I do to avoid doing things that are important
surfing wrong planet
watching telly
eating
cooking
housework eg washing dishes, clothes, vaccuuming!! !
What does everyone else like to do for fun?