Step One: In the "what wonderful thing shall we throw as the bridal couple leaves the church" discussion is: Check to see what the facility allows.
Step Two: If you use something that isn't biodegradable, you must have someone(s) pick it all up. A time consuming and no fun job that could loose you a friend and make relatives wish they weren't.
Below is a list of things that have been thrown and some thoughts for you to consider.
Rice has been a custom since back when it was meant to give the couple good luck with their crops and provide longevity. The debate about it making birds explode has "opinions" and "facts" on both sides. The real fact is in this "litigation happy" world in which we live, rice might cause people to slip and fall down steps or on hard surfaces and most churches and reception halls have banned it's use.
EcoFetti is a brand name for environmentally friendly confetti. It comes in several colors and resembles old fashion artificial snow. It will melt away (so they claim) in the first rain or may be hosed away. I don't know if it stains clothes.
Dried lavender buds don't sting if they hit the couple and they smell wonderful. You may still have to pick them up if there is a large quantity.
Rose petals can often be bought from florists and if you agree to use "seconds", the charge is much less. Rose petals can also be freeze dried. If it's raining/snowing, the colors might stain a wedding dress.
Butterfly releases are expensive plus the butterflies are often not ready to fly and throwing lots of insects that immediately drop to the ground and then are trampled is just wrong.
Bubbles is the new rice because it has been used quite a lot recently. Some wedding guests simply refuse to do this, some aren't very effective but if you have a crowd that is willing, it can be pretty. It doesn't work well on windy days and it may stain the material on the wedding dresses.
Seeds sometimes are an alternative to rice. Bird seed, grass seed, pumpkin seeds have all been used. They can also cause slips and as with rice, can get stuck in the brides elegant hairdo, down her dress and undergarments and make a miserable bride at the reception.
Doves being released is another beautiful Hollywood idea that might have a bad outcome in real play. Domesticated white doves are not always adaptable to all habitat and would find it difficult to survive especially if it is already cold outside.
Noisemakers such as kazoos and whistles can be fun if you have a crowd that likes to make noise. Obviously, you don't throw them, you play them. They can also be a memento of the day.
Streamers on sticks is visually pretty and can be waved instead of actually throwing things. Wooden dowel rods with ribbons on the ends can be handed out as guests are leaving the church. A friend can gather them or you can have guests take them to the reception and then home. The ribbons can be in the wedding colors. Realize young children will eventually use them as swords.
Helium balloons in the wedding colors can be released but there is the environmental debate as to the damage it does to animals/birds/and as trash once they come down. Also, Mylar balloons should NEVER be released outside as they can get into overhead electrical wire and substations causing outages.
Sparklers at night weddings is said to be beautiful. My concern would be the flammability of some clothes (especially the wedding dress and veil), getting them all lit at once and the disposal afterwards. Children would have to be closely supervised.
Fireworks would certainly be showy at a night wedding. It would involve an experienced and talented (perhaps licensed) person to do this, check to see if you need a permit, and you'd have to consider flammability of the area. Setting a two-hundred year old church on fire might take the zing out of the wedding. Both sparklers and fireworks are against the law in California (and perhaps other areas.)
LED lights also are fun if waved by the guests at a night wedding. They can be a memento for them to take home and are safe for children. Cost might be an issue but there are little ones on key chains and they come in many colors.
Little bells can also be a memento and there is the wedding reception game that every time someone rings their bell, the bridal couple must kiss. Some couples have their names and the wedding date engraved on each bell.
Dollar bills could be thrown but would need to be picked up by someone who would return them to the couple and it would take some planning to accomplish this little trick. It is a custom in some cultures as is throwing coins.
Dried leaves and herbs would be pretty in the fall and are no big environmental threat. Large quantities would probably need to be swept up.
Maple tree seeds "helicopters" float down slow and softly. They would need to be gathered in the spring and dried.
Some brides have opted to no longer have things thrown because it can get out of hand, sometimes hurts, messes up their hair, can damage their dresses, and is an added expense.
The most novel idea I've heard about is the Redneck wedding where they throw punches. Environmentally friendly as long as they don't draw blood.