Levels which let players run through non stop are never fun anyway. When I design single player levels, which I spend a lot of time doing these days, I make sure that I place lots of puzzles around the level, and often force players to explore by presenting them with multiple routes (or sometimes I just go insane and make the level totally nonlinear so it resembles a sort of gigantic multiplayer level...), blocked passages which open up from a far away trigger crate, and elaborate secret areas. There are often unique challanges in the levels I make too, such as moving platforms or "Pits-of-doom-and-certain-instant-death." I call those Podacid. Catchy, isn't it?