Inventing is not really what it is lived up to be
The problem with it is that there are tons of widgets/toys that serve little use, but only 4 (and simbot) "big" inventions that serve a purpose.
The first is the floor hygienator which gives moodlets like "minty breath" or "nauseous" when walked over. IT can give both good and bad moodlets. IT boosts hygiene by about 10% of the bar when walked upon.
The second is the harvester, which collects collectables/harvestables/spills/messes around you.
The 3rd is the miner, which can be used to make holes to explore, and uncover gems/metal/meteorites.
And then there is the time machine, which going into results in +25 moodlet or -25 moodlet for 4 hours, and sometimes some simoleons, a rare book, or other trinket. You can also return with a child, or as a supernatural life form.