I got a lot of tips for this from the 2018 challenge
https://www.carls-sims-4-guide.com/forum/index.php?topic=30320.75Created an Alien as they can transmute crystals, metals, and elements as well as reviving dead aliens. In the end I wasn't lucky with this and only gained three or four items with the alien skills. I made her cheerful, like the outdoors, and kleptomaniac. The last was so that I could steal the insects needed but soon realised that I would waste too much time levelling mischief and waiting between swipings. Collector aspiration as this doubles some finds and supposedly helps find rarer items. Maybe Geek would have been useful. Joined good timers for Rally the Troops and took it over. Used swiping objects as club activity whenever I needed cash, had to be vigilant though as sometimes my Sim would go for low value items.
Main strategy was to aim for completion of categories under my control/ the less random ones and otherwise seek a range of types which would contribute to the multiplier as well as number of unique collectibles
collections breakdownFrogs - 18 - Got unlucky with duplicates so breeding was limited use but ended up with a fair number
Gardening - 71 - worth pursuing for sheer number of collectibles available even though completion wasn't on the cards. Micro home and skill boosts helped max level so could buy all seed packets, plus visited the stalls in Henford on Bagley (thanks to tips from folks in the spring gardening challenge)
My Sims Trophies - 14 - Dug them up. Some available to buy in Paddywack's Emporium in Magnolia Promenade
Metals - 17 - Dug them up, transmuted, bought some from stalls at the Flea Market and Art District
Crystals - 23 - (one left to find) Dug them up, transmuted, and bought a lot from stalls
Elements - 14 - sent all the metals and crystals off to the council, transmuted. Forgot they come in post following day so sent last batch off too late
Postcards - 14 - This one is most annoying as it would have been complete had I remembered to uninstall My First Pet Stuff pack which adds some postcards your hamster/ whatever can send you
Fossils - 14 - Dug them up, may have bought some at stalls
Microscope prints - 7 - Levelled logic to 8 by using machine so could use samples from fossils, crystals, and plants as well as the prints just from using. Didn't upgrade machine which might have helped
Space prints - 1 - Stargazed once in Myshuno Meadows while waiting for its many collectibles to spawn
Aliens - 7 - found on Sixam and space missions. Could revive dead ones which added one I think
Space Rocks - 1 - none of my space missions resulted in a space rock so the only one collected was bought from Paddywack's Emporium
Fish - 36 - fished in a variety of places, nothing special here
Insects - 8 - visited Granite Falls for a few hours that covered both day and night to maximise chances. Wasn't terribly lucky but eight is pretty good considering
Geodes - 6 - COMPLETE! Two trips to Sixam. Maxed rocket science with help from skill boosts, wasted time on another install when I could have levelled just with space missions and got more chances for collectibles but only thought of that afterwards
Experimental food photos - 20 COMPLETE! bought a chef station, hired a chef and worked through the experimental menu. Worked on Simmies, Voidcritters, and Holiday Cracker Plushies while waiting for the food. Thanks to GlazeyLady in the 2018 challenge for this strategy
Voidcritters - 20 - COMPLETE! Just bought booster packs until collection was complete
City Posters - 6 - boxes around the city
Snow Globes - 15 - COMPLETE! Bought whole set at flea market. Another tip from GlazeyLady is to travel to another lot once you've emptied the table then return to the flea market and another set of collectibles will appear. I moved between the flea market and arts quarter, which also has a table, several times until all that turned up was toys
Holiday Cracker Plushies - 8 - COMPLETE! Bought them until complete. Got hysterical a couple of times so more dangerous than I thought
Magic Beans - 6 - COMPLETE! Once max gardening skill bought rare seed packets until I got the set. Also got other rare seeds for the gardening collection and once planted I got the forbidden fruit for it too
Ancient Omiscan Artifacts - 1 - dug in archaology piles until I got a couple of potential artifacts. It was risky but I didn't want to waste time. Luckily one was legit
Seashells - 3 - beachcombing in Sulani. Generally when I was tired of grinding something else
Buried Treasure - 4 - fishing in Sulani. A massive variety of fish plus the chance of treasure made it efficient
Magical Artifacts - 13 - this one really surprised me, I thought I'd be able to pick up a couple of basics at Casters Alley but two visits provided 13 items. Think I also picked up a metal from a Casters Alley vendor but crystals and gardening were duplicates
Simmies - 23 (one missing). Bought the machines to use at home and every time one jammed I just moved to another machine and deleted it. It was so close I was buying them until 07:59
Village Fair Ribbons - 1 - bought an egg from Kim on the way to the fair and submitted it for a participation ribbon
Message in a Bottle - 12 - was told there was a 3am respawn on these and managed to time it well first time as there are six spots to search across two beaches. Second time went less well but still collected more than expected
Missing collection typesDecorative eggs - am unaware of how to get these outside of holiday tradition of egg hunt
Sugar Skulls - not sure how this one works. I was tempted to kill a visitor to try it out but time contraints meant I prioritised other things
Feathers - I had assumed this wouldn't be possible without a pet, however I read that it can be done with a befriended stray - would be interested if anyone has achieved this
Omiscan Treasures - didn't think it was worth the time
Moonwood Relics - thought I might get these by exploring the tunnels in Moonwood Mill but wasn't lucky
Hope this is helpful, let me know if I've not explained anything.
