Disclaimer: I haven't actually looked at your game, but from the description feel able to give input anyway.1. How do i fill the environment in racing games like in my case? It feels so blank and empty with just 1 sky-box. All i can think of is some buildings.
What might you find in a real environment? Pick a location for each track in your game, and fill it with things that would be in that environment. If a track is located in the city you might have buildings, trees, mail-boxes, chairs & tables, pedestrians, etc. A country track might have some cows or horses, plants, farm equipment, etc. You can also make your skybox more interesting by adding planes, birds, blimps, whatever as appropriate.
2. Are there any tips and tricks for making levels / tracks fun? All I could think of was like turns and jumps
Think about what players can do in the game, and then design the tracks so that all of the skills are used at some point, preferably spread out. You also want some variety in the experience -- you might have a series of tight corners that really test the player's ability to steer and control their speed, followed by a straight where they really get to accelerate, and then some wider sweeping corners where they can still overtake if they take care but need to watch their steering. Things like jumps, destructable obstacles, or terrain that can slow the player down might add more variety.
Where possible, also offer the player choices: do they take the risky but shorter path, or the longer but easily driven one? Do they go over the rougher terrain to save travelling as far, or all the way around the track? This can be especially effective if there is a choice of vehicles which handle different actions better or worse.
3. I have put fence around the tracks so that player / AI don't fall off. Is this a good idea? I wanted floating tracks and hence this issue.
You could consider using this to influence difficulty -- easy tracks (or easier
sections of tracks) might have a fence, whilst more dangerous tracks or sections won't. You could also have fences that can take one hit and keep a player one, but then fall off and potentially prove dangerous for following players.
If there is no fence it might become a tactic to try to hit your opponents and knock them off.
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