To start bonus level you have to pick up 175 gold bricks and unlock Clark Kent. You will find entrance to this level in the centre of central island in park (location is marked with golden brick). To start this level use gold bricks to build a gate (picture) and pass through it. Finish this mission to gain lots of coins and one gold brick.
In order to finish this level you have to pick up 1,000,000 coins (multiplier from red bricks are off) so you must destroy almost every object on the map. Some of them are quite hard to find and level is fragmented on 3 sections (islands linked by bridges). Below is list of coins harder to find, easier one aren't describe here so destroy every object on your road.
1 - Electric fence with car - you will get here a little bit later - Robin has to use hazard suit to clean chemicals spots and Batman must cut off the electric power using electricity suit.
2 - Roller-coaster - you can jump into it and quickly get lots of coins.
3 - Rockets - use Batarang to knock off them, destroy them on the ground and crush the scaffold. When big rocket fall down destroy the flap and you will get access to hazard suit for Robin.
4 - Metro entrance - you can pass through it if you have Batman's sensors suit (you have use invisibility). When you are inside move to the right side of carriage and pull lever. When door opens jump Batman out of the carriage and pick up coins. There is time limit for this sequence but you can repeat it at any time.
5 - Plants - to destroy them you have use water from hazard suit and when they open jump onto them.
6 - Glasses - you need bat suit to destroy them.
7 - Sensors suit - you can get him using magnetic suit (look at #13 - then move to the pillar on the left side of the bridge, get to the top using blue pipes and fly on the right pillar) or you can use Superman.
8 - Power supply - you have to fill three receptacles using electricity suit. You will find electric current in #1, #10 and #20.
9 - Power station - after turn on power in #8 turn windlass and when power gets to the letters in #11 destroy them and build "L".
10 - Receptacle - use it to gain energy and supply part of electric device in #8.
11 - LEGO inscription - you have set LEGO inscription. To do it turn left valve on E, press the ground button when G shows up and turn the second valve on O. How to get first letter look at #8 and #9.
12 - Penguin's hideout - destroy two penguins standing near entrance and Penguin will show up. Switch character on him and use his bomb on little igloo nearby.
13 - Palm and lion - use Penguin's to destroy fence, jump on the lion and run along path of coins. To destroy palm you have to Batarang towards the top of the tree (destroy it and you will unlock magnetic suit).
14 - Factory - clean all chemicals using water, fly Superman on the top of the factory, destroy all parts and gold top and jump on little red chimney. Get down and turn the valve to unlock Joker's robot (you can use it to destroy black bricks under horse's statues and Batsign (#18).
In order to get to this location you have to use metro (#4) or jump over the abyss on the lion (#13). On the left side of the church (#15) is phone box where you can change Clark Kent into the Superman. On the street north of the church you will find electricity suit (before you get it you have to destroy car's wrack).
15 - Church - you can destroy it completely (e.g. using laser) but you have start from the roof.
16 - Residence - turn windlass and when residence go higher destroy silver foundations (use Penguin). After that destroy it starting from the roof.
17 - Metro entrance (look at #4).
18 - Batsign - you can destroy it using Joker's robot (look at #14).
19 - Batmobil - enter it and path with coins will show up. It's over with jump so if you want to get this coins you have to use valve near bridge to lift the bridge.
20 - Crane and receptacle - use Batman's electricity suit on the receptacle and power suit will show up.