Zoo Keeper

Here's a very low-benefit trick for Zoo Keeper:
start the game and immediately crash into an animal. If you have never jumped before this silly action, you won't lose a life: the game will simply advise you that you need to jump over animals to avoid them and round 1 will start again from its beginning.

This seems actually to depend on a time limit. You can get away with proceeding clockwise around the cage, waiting at the spot where the first mug appears,
then moving to the second mug's spot. Immediately after you collect the second mug, run into an elephant. The way this usually works is that you have to jump two
elephants on your way around, but the game still gives you a free life. Best of all, you get to keep the points you scored!

An obscure trick: On the coconut boards, when you jump onto the top ledge, you get some points. Jump off the ledge then onto it again: you'll get twice as many points.
And again, for four times as many. The bonus keeps doubling until 300K per land. The best board to try this on is the board where the ledges are invisible. It's kind of hard to jump on then off, but so few coconuts are thrown that you have lots of room for error.