Characters
There are a number of different other characters that you can unlock. You get a few of them to start off with, and the others ones can be earned through various tasks, such as collecting all three coins on one of the levels, or rating the game. Tap the ones with the gold locks to see what you have to do in order to unlock them. The ones with the silver locks can only be unlocked with the pay version of Geometry Dash.
Replay for new obstacles
Switch back and forth between levels. If you are getting stuck or your score is getting even worse from game to game on one level, switch to another level and try that one, even if it's technically harder. You might even see some new obstacles that will help you when you go back to the old level.
In addition to the jumps and the falls and the spikes (the usual obstacles), also be on the lookout for collectible goods, as they will unlock new characters for you (one example of this being coins). Plus, watch out for unorthodox obstacles such as lava. You can sit in lava for about one second; any longer and you will end up dying.
Various Gameplay Tips
Tip 1
Enable music and sounds enabled to develop a rhythm to know when to jump.
Tip 2
There are large golden coins hidden around each level. Memorize their locations to collect them in future playthroughs.
Characters...