Aww that's very sweet of them!
Personally, I wouldn't expect anything when I go to a wedding, but typically you get a welcome drink either before/after the ceremony, or both (!), plus wine on the table and fizz for toasts. That's what we will be providing.
I thought about putting money behind the bar, but we just don't have the budget for it. If we can stretch to it, I'm going to try and arrange for evening guests to have a welcome drink.
The issue with putting money behind the bar, is there's always that one person who knocks back 3 double GnT's, before your teetotal friend can get her first diet coke in!
If they are insistent, maybe see if they can cover/contribute to the costs of the welcome and table drinks?
I considered drink tokens too, and it is do-able, I just couldn't be bothered with the faff! X