Looking to get in some modding practice, and people have insisted that tuning mods are the place to start.
Problem is, anything I think of that I'd want to adjust in Sims 3, someone else has done it better than I would have.
The only things I ever feel the need to complain about that tuners can solve are Sims taking too long to do things (solved by the Longer Days mod), and pregnancy being too short (solved by Longer/Shorter Pregnancy mod).

Anyone have some ideas on what I could tune as practice? I intend to eventually move on to script modding, and making traits and such, so I need to get as much practice as possible. As such, all suggestions are welcome.

Another idea, would it be possible to convert some Sims 2 mods to Sims 3, assuming it's something that might apply to both games?

Nope. The sims 2 doesn't use XML. And the file that does uses something similair to it won't work out with the sims 3 as they are completely differently coded. (Unless you want to make a new XML tuning mod and change the script). Unless you meant a different kind of converting.

Traits however, is going to be hard, unless you know some #C language. I would start looking around gameplaydata.package in the path: C:\Program Files (x86)\Electronic Arts\The Sims 3\Game\Bin\Gameplay (Unless you installed the sims elswhere, then you should search for the game/bin/gameplay.

There, you want to export the XML of your choice, open up a new package and import the XML file. NEVER edit anything IN the gameplaydata!

My first experiences with tuning mods were simply "No Autonomous..." mods for myself and my family. Things like No Autonomous Fishing, that sort of thing. Find the files in the gameplaydata, export, open in a new file and make your edits. Then test in game. Even if it's not something you actually will use in your own game, it's good for practise purposes!