Remove every obstacle you see when you have a free builder and have enough gold/elixir to do so. Removing obstacles sometimes give gems. Keep in mind that if you remove every obstacle, the trees will come back over time but the rocks will not, so it's a good idea to get rid of the trees as soon as possible because that'll free up more space for new trees to come back so you can get more gems. Also, complete achievements. Some achievements are not that difficult to get and give you a ton of gems. An example of this is the achievement you get for getting 1250 trophies. It gives you 450 gems. If you already have some gems and/or save some afterwards when you get this achievement, you'll be able to get a third builder for free.