Abus and Kryptonite folding locks are a better go-to than a u-lock. They offer the same security but can be locked to a wider variety of things moreso than a u lock. Perhaps consider a chain loop style lock, especially if you've got panniers on the bike. Chains are the easiest to lock stuff up with and have some of the more secure locking systems. If you don't run a pannier or bike bag the folding locks have the best holders that come with them. They'll mount where you'd normally have your water bottle. Also they'll usually include a method to attach the holder on an area of the frame that doesn't have any bolts for mounting, if that makes sense.