It depends on the size you're looking at first off.
If it's a 64gb, probably just windows. If it's a 240, like mine, then windows, a bunch of heavy apps, anything that runs at startup, and maybe a game or two.
After that, just think what you use the most. It's nice to have heavy programs on there, like CAD software or simulation software. then office or other things like that are nice, like other "core" apps.
Games are probably the last thing to put on it unless you play a game a LOT.
Remember, if you play a game, say a steam game, a lot for a month or so, you can always put it on the ssd for that time then take it off after and install it to a hdd. nothing is set in stone.