[f300]

Idea is very good but I don't think it can go very far with cities, there are easily 20 "interesting" cities in each of the "big" EU countries. Only investing in public transport tickets (which also usually expire) is a big task (and they aren't usually easy to get online but one of the easiest thing to get at the destination).
With SIM cards you should be able to easily manage 5-10 countries; most of the info about plans and options are available already on prepaidgsm.net, you'll need to keep a db of yourself with those (also with expiration dates and credit/options on each card).
You can also do some partial refunds if people send the cards back to you after they're done (postage is like 1-2EUR for an envelope throught Europe so this shouldn't be a problem). You can even do more complicated stuff: card comes back with some money on it, you put it in your database as available with precisely that credit (make sure you deactivate any recurring charges), somebody buys it paying for the included credit then you refund some of the money to the guy who sent you the SIM back with the remaining credit (of course minus some comission that would cover all the costs involved). Or you give them store credit.
Also you could use your customers as suppliers as well: if you always run out of more exotic cards (for India for example) you could pay back your customers if they return more cards than you gave them. For example you give one indian SIM to a customer who gets once more for you while it's there, then he returns both cards to you and you give him some compensation.