Despite any desire to lay low to preserve the timeline, I can't see Clark not helping people. At least he's not in any danger of killing his own grandpa (nor becoming his own grandpa). Not that Clark would kill anyone, but he could, e.g., save the first husband of a woman who, were she widowed, would have married someone else and given birth to, say, Sam Lane' mom.
So Clark isn't in danger of not existing, but Lois is. And both of them could end up in a temporal paradox if they cause things to happen that prevent them from going back in time in the first place. Of course, then you get into a discussion of exactly which flavor of time travel you're using and whether you can get such paradoxes.
There are a lot of possibilities here. But it's very easy to make a terrible time travel story. For me, the number one thing that I need to enjoy a time travel story is consistency. Use whatever flavor you like (time loops like in Harry Potter or alternate timelines like in Back to the Future, etc.), but pick one and be consistent.
If Lois and Clark were stuck back in time, I could see them leaving a message for H. G. Wells somehow that he might be likely to read some point in the future so that he could go back and pick them up.
