Only 3 days in a city? That's all?
Food and Drink - Eating and drinking can sometimes run up half the cost of the trip. Grab the free breakfast at the hostel, then while out sight seeing, locate a local grocery or farmers market. Purchase all the picnic essentials then take it to a local park or ruined castle. Drinking at the hostels is almost always cheaper than at the bars by a large margin. You'll find the other savvy travelers do this as well. I found myself starting the night at the hostel's bar then heading out as a group to a pub to socialize with the locals. As stated above I tend to splurge on restaurants every 3rd day. Of course this is only when you're trying to be frugal. The alternative is to eat at restaurants the whole time which is extremely easy since there's a cafe or restaurant on every street corner.
Picnic at a ruined castle in Bacharach
Fresh fruits & vegetables at the Viktualienmarkt