We've just bought a cheapo booster seat and a wipe clean table cloth for the dining table. 18mo will now sit happily for over half an hour playing with felt tip pens, with no worry about the state of the table afterwards or falling off the chair as she tries to climb down. It is, quite frankly, revolutionary for the princely sum of about £25 all in. I am in love 
When she was little, the walker/jumperoo was an absolute lifesaver, but that one is reasonably obvious. We also had a £5 old-fashioned plastic play arch that seemed a million times better than the modern ones.
What small or odd things completely changed things for a while for you?