It all depends on the zone you're designing. The first zone is usually quite small, and things get more complicated as you progress. You want to ease the player into the game mechanics before adding more advanced level elements.
A good idea is to look at previous level maps, and draw the path through the level as a straight line with branches. This should give you a good idea about how complicated to make things
The later Genesis levels were much bigger than the early Sonic games, but they were also detailed. It's tempting to create a massive level and then try to fill it with stuff. Don't. Instead, start with a small area and expand from there. Stick to a theme (such as trees or caves or whatever) and use that to build your ideas and your level.
Oh, and don't put in too many death pits. Those things suck.
The quick version:
Look at how levels in other Sonic games branch out
Sketch your level out on paper using a similar branching
Make a big list of ideas for your zone. Gimmicks, objects (trees, bushes etc), badniks and so on.
Work these ideas into your zone. Don't try and cram everything in. If it isn't fun, scrap it. Unfortunately a lot of game development involves throwing out stuff that doesn't work.
Get people to play it to get honest feedback. Then improve.
Ok, that was far too long, but I hope it gave you some help.