1. MID SIDE EQ / LEFT RIGHT EQ - You can split your into either Mono vs Stereo or Left vs Right and EQ each separately. This will give some VERY cool stereo effects. On the mid-side EQ try and push some of the side signal to create more volume out wide. Same approach for L&R, process them separately and use your ear to see what kind of spatial effects you can create.
2. STEREO DELAY - Using a very short delay 2-10ms will create a quick repeat on the side which can give the effect of width. You can also play with the HAAS effect (delay L&R channel from one another) to add width.
3. TWO PANNED REVERBS - On two empty return / aux tracks, set up two different reverbs. Pan one LEFT, one RIGHT. Send your signal to both. Mess around with different decay settings on each to manipulate the sound and width. You can also put EQ's after these reverbs to clean up unnecessary low end.
4. STEREO ENHANCEMENT PLUGINS - A few favorites of ours are Bass Kleph's Easy Stereo & Infected Mushroom's Wider. These two do a great job spreading the signal, while maintaining a quality mono sound. What are your favorite enhancement plugins?
5. PARALLEL DISTORTION + FX - This is a fun one. Parallel distortion in general is a great way to add character to a sound. Send your signal to a bus with the distortion. You can also put other FX after the distortion like widening plugins, chorus, flanger, etc. Don't forget to clean up low end with an EQ on this track.
**Keep in mind to always check how these sounds are in MONO after spreading them. You want to make sure you're not losing too much of the signal in your mono mix.