After you complete the game, you will need to work towards the various "Destroy X amount of item Y" achievements.
For the gas stations, I suggest replaying Level 3-3 (Mountain 3). It has 8 traffic signs, which are all relatively easy to break, as well as 8 ad signs and 1 gas station.
Actually, what I did was:
1) work on 1000 crops on track 1-3 (following BADGUYSKI's guide).
2) work on 1000 fences on track 3-1 (following PuritanSoul's guide). This level also has 4 traffic signs, 6 ad signs, 3 barriers, and 1 gas station.
3) work on 500 traffic signs, 250 ad signs, and 100 gas stations on track 3-3 (my guide).
4) work on 500 barriers on 1-3 (following x NILL RAYDEN x's guide).