What type of icing to use?

It is DS 1st birthday next week, and I am going to try and make a cake...I use the word 'try' because I cant actually make icing...never works out (except my royal icing for gingerbread - can do that).

Anyway I have grand plans to make the cake and then ice all over in green icing (for grass) and then use some lollies to make a racing car track (I know, I might be going a bit far on my first attempt)...but what sort of icing do I make to ice all over the cake? I'm thinking it needs to be that softish type...but not sure that that is called.

As you can see this could all end in a big colourfull mess...so any help would be appreciated!

Thanks
FG

Easiest Buttercream recipe I have is:

250g Butter
250g Icing Sugar (you can add more if you want)
1 tsp vanilla
1 tsp-tbsp Milk

If you want it to set easily you can use a tbsp of meringue powder (like the stuff in the egg to make the pavlova) but I haven't done without and it's not necessary.

Whip the butter till soft and slowly incorporate the icing sugar about a large serving spoon at a time (I have a big metal spoon). Just taste and add as you go. It's important not to add all the icing sugar at once so you don't get that grainy texture. Then add the vanilla and milk once you've got the consistancy you want. Then transfer to bowls and colour.