ESD (electro static discharge) is almost always an issue in this instance with the lcd and keypad. Putting the lcd display 2cm away from the window can help things in terms of susceptance but doesn't look particularly good in cosmetic terms! The general solution is to make sure the 0V on the lcd module is well connected to the 0V on the rest of the circuitry and the metallic enclosure (if you have one). Also you don't want to have high speed digital signals going to the lcd if you want to pass the EMC requirements. On my equipment where I have lcds, I haven't had to go to extreme lengths to keep them quiet - again, the usual 0V bonding and the lcd is run off port pins with pullups (on a 8051). No filtering or suchlike. As to your 3m cable with TTL level serial - for CE this cable will be zapped so you want filtering on these signals for EMC and ESD. A bit of R & C and some ferrite beads will take care of most cases. Obviously more R & C is going to slow the edges of your digital signal so it becomes a tradeoff of speed. The R & C also have the effect of 'knocking the edge' off the ESD spikes. Zeners, tranzorbs or MOVs (varistors) are also a good thing. I think it would also be suggested to use a MAX202 for your serial instead of TTL level - these have controlled transitions which will ease your EMC problems. TTL level signals are really only good for between boards - don't send them out into the real world (at least not without some protection).

I find it's always educational to look at how the larger companies have incorporated EMC defenses into their products - look at an Apple board!