If you want your guests to have a good selection of drinks, but you don’t really want a completely free bar, here are some ideas to help keep (nearly) everyone happy:
Limit the choice to some key drinks that have wide appeal, and select a lighter or heavier choice in category. For example, select a few nice wines, a couple or signature cocktails, and couple of types of beer (lager & bitter / stout / ale).
If you’d like to offer more of an open bar, you could arrange with the venue to have an free bar to a certain amount (of money), and then switch to cash for those who want drinks after that point. You may need to keep a limited selection of drinks available all night e.g. a red, and a white wine, and a couple of beers.