My two cents is don't bother. I didn't when I got married, having worked in a wedding venue I saw time and time again the vast amount left behind on table and it really put me off. It wasn't dependant on how 'good' the favour was either, wonderful homemade jams, personalised coasters, hangover kits, tea towels, some great stuff left.
The spend for the bride and groom is high, but per guest it is low so they don't really get anything special, but it's a fair amount to add onto the wedding budget elsewhere.
There are success stories like further up this thread, but my experience is that's the exception to the rule. Also, these couples may have bought 100 plants or baubles and hear about them from 10 guests, which is still lovely, but not a high rate.
My advice is if you feel you should, make it immediately consumable, those always had the most success that I saw, people arrive at the table and scoff the chocolate, very little waste.
Lastly, on these threads I always see people recommend charity pins. Don't bother with the pin, just donate the cash amount, most people won't wear the pin (these were also left on tables alot) and the charity keep more money. £100 cash donation = £100 cash for charity. £100 pin purchase = £40 cost for pins £60 cash for charity.