There is indeed a mod for that. Twallan's DebugEnabler. Its current location is:
http://nraas.wikispaces.com/DebugEnablerWithin its host of gleefully powerful powers is the ability to spawn specific RGM and insects directly on the ground for collection. It's pretty menu-intense and easy to get lost in the multiple nested menus, but if your sims are growing elderly waiting on that pink diamond or sunstone and can't seem to find them even with a collection helper, and you're dead-set on cheating, this is the way to do it. It's not even a core mod.
As far as gardening goes, TwoFtMama at her Naughty Sims Asylum (
http://simsasylum.com/tfm/) has a number of gardening mods in many different flavors from convenient to downright cheaty; look for "Plantastic" over there. Also, search around
http://ModTheSims.info to find things like "no more toetap" and "no more routefail tantrum" to help speed up automated gardening, particularly if you're using multiple gardeners and/or a complicated gardening plan. Also on ModTheSims, there's also a magic fountain that auto-waters your whole lot, and a magic gnome (possibly outdated) that can force the whole lot to bloom at your whim so you can rapidly achieve perfection and maintain it with little effort. DebugEnabler can also cause individual plants to bloom, but it takes a few clicks through a menu full of unrelated options.
As far as fishing ... a level 10 fisherman who hasn't even beat any of the challenges or learned any bait can catch deathfish without bait, so all you really need is the skill. You don't need Angler, Gatherer or Loves The Outdoors (granted, they
help), and you don't need to read the fishing books or even learn the bait chains to catch the exotic fish. You just need a sim to put the time in. If you want to reduce the time your sim puts in, get them the Fast Learner lifetime reward, or do what I do: send your vampire out after dark while hopped up on a Skill Gain drink. Also, make sure their mood is up while they're out fishing. The simplest elixir of bliss will work wonders here, and a potent invigorating elixir will ensure that they can stay up all night and all day fishing. You'll have your mastery in no time if you work with what you've got!