Great thread!!
Summer lines are a fun time. It all comes down to steepness and texture of what you're riding on. Grass works but you need some friction to really get moving so a couple rocks here and there is ideal. I would recommend mowed grass and the steeper the line, the better, since summer conditions are generally slow.
1 - Stairs are honestly the best and most fun summer lines (escalators too).
2- Grass/ woods/ forest hills come in second.
3- Sand comes in third because it's actually surprisingly hard to ride on sand and you will need weird stuff on your bases to get the proper friction. If Andy Parry could chime in here on how to effectively prepare your equipment to ski on sand dunes, that would be much appreciated. The last time I tired skiing on sand I was barely moving even though the hills were steep.
https://www.newschoolers.com/videos/watch/823110/Skiing-down-a-pile-of-rocks
https://www.newschoolers.com/videos/watch/823111/LINE-Traveling-Circus-3-1-Attack-of-the-Sand-People 
