Answer: It's a Georgian/early Victorian mourning ring. The initials belong to the lost loved one.
They were typically made from gold (18k+) and enameled in black. Yours looks like it was made around 1820s-40s.

Answer: It’s a pressure relief hole. If you dropped it, instead of exploding, the hole would pop out.

Answer: that’s a broken megalodon tooth.

Answer: It obscures the car while being test driven, so the yet-to-be-released models are not photographed and published. The random "wave" patterns effectively hide the body contours and lines.

Answer: it's a residue of a rocket launch. It's called Twilight effect. It was done right at sunset so the sun lit the vapor/exhaust trail from underneath.

Answer: voice recorder, doesn't transmit using wireless though so whoever placed this thing, will be back. If this was in your home, chances are there are cameras as well.

Answer: Black bobcat.

Answer: that's two worms making more worms.

Answer: a portable men’s urinal for bedbound patients. It is used to pee when you cannot get out of bed.

Answer: they're childrens' training scissors. Like for pre-schoolers. The extra holes are so a grown-up can co-scissor and help the kid.

Answer: a WWII one man bomb shelter.

Answer: it goes over shoes to give grip on ice. They are called yak tracks.

Answer: Secret Industry by John Pym: The stairwells and lift lobbies of multistorey car parks are not usually areas where people pause to admire the architecture, so John Pym used this to his advantage. The model walkways and ladders replicate their full-scale counterparts and are a playful way of suggesting an alternative, unseen service industry at work within the building. Superficially mundane, some people may use the car park for years without being aware of their presence, while others will notice straightaway. The artist sees it as a reward for those who look more closely at their surroundings.

Answer: It's a snowman kit.

Answer: This is a "tracker pebble" used for tracking longshore drift. The pebbles have a serial number, are buried in a location and then refound further down the coast in the shingle by metal detector.

Answer: the glow is related to a commercial cannabis growing operation. That's the color of grow lights.

Answer: Bedsore prevention. You would refer to the two hour intervals as a turning schedule.

Answer: a Voodoo altar

Answer: it's an automated delivery, possibly for food.

Answer: it's a geocache. Judging by the size, it'll likely only have a list inside for people to sign that they found it.

Answer: In Denmark they are called a "gadespejl" It is an old invention, basically a mirror that made it possible to view the street from the comfort of your couch.

Answer: Squid eggs.

Answer: it's called a Papst (german for Pope). It's for puking.

Answer: a mineral mine. It's the world’s largest lithium deposit.

Answer: The Lucky Iron Fish. I recall hearing that iron deficiency was a serious problem in developing countries due to improper diets that consisted of mostly pastas and rice, which are a poor source of iron.
The solution was to boil chunks of iron with food to increase the iron content but many were skeptical and hesitant to cook with chunks of metal in their food. The iron was shaped into a 'lucky fish' that would provided addition health benefits when you boiled water with the fish in it.

Answer: It means do not scare the flamingos. Many people would cross that sign to scare them so that they could fly and get a picture.

Answer: we call em bee hotels, like many people already mentioned, it is for 'solitary' bees (solitary because they don't love/work/do in hives, but clearly, they live close to one another in this setting) although it can also attract many other animals. These things help the ecosystem.

Answer: it is a rain chain, an alternative for traditional downspout of a gutter system.

Answer: Pepper Spray Paint Ball gun. Technically it is a paintball gun, however the paintballs it fires have pepper spray powder inside of them. It's less than lethal defense.

Answer: Float Plane Mover. This truck's job is to move planes up and down a boat ramp into the water. Backing up an ordinary truck+trailer combo down a boat ramp is not easy. With this vehicle, the driver can look straight forward down the ramp rather than in his rear-view mirror, and since the steering wheels are at the back it's easy to align the truck's bed with the plane. Most trucks only load at a boat ramp once in a while, since this one does only that, it makes sense to design it specifically for that job.

Answer: The Quester, the private submarine built to salvage the Andrea Doria, now in need of salvage itself.

Answer: the bleach oxidized the metal parts of your sink.

Answer: The Black Drum fish fossils. The Black Drum had a short deep body with a high arched back, but a flattish belly. The jaw teeth were small and pointed, but the throat was armed with large, flat, pavement-like teeth with which the drum crushed shellfish, for food. This characteristic separates it from the weakfish and the king fish.

Answer: the remnants of a scientific clam survey.

Answer: It’s a refill container for a marking tool used in sewing. The tool dispenses a fine line of powdered chalk when drawn across fabric. I have some in several colors.

Answer: to transfer groundwater to rivers because of the water level. Since the early 1990’s the groundwater level in Berlin has been rising. Due to lower water consumption the abstraction of groundwater has diminished and the city now has to cope with an extraordinary high level of groundwater.

Answer: Former employee, it’s more than likely from their gloves they use to put the meat on the grill.

Answer: contrails from a passenger plane. Being close to sunset, the vapor clouds are taking on the same color as any other cloud would have under the same late-day sun.

Answer: small scale meth making device.

Answer: Ibuprofen.