Vary the pitches so the toss, batting first and huge totals don't spoil the game. There, Tests made more exciting. Anyone watching the 1991 Headingley Test or either of two Tests which were very close in 1998 between West Indies and England will know exciting Tests and that's just three Tests that readily spring to mind.
Tests can be boring because the pitches are too flat or the games are one sided, that's because West Indies, Bangladesh and New Zealand aren't at the same level as other sides and the pitches can favour batting first too much or are so flat it's a runfest with no prospect of a result. Reducing the number of days is a daft idea, you'll just end up with more draws unless you do something to produce results faster.
I think people need to stop worrying that T20 is going to ruin cricket and trying to make other formats compete with it. I suspect the fad will soon wear off, remember when the yanks were trying to make football more exciting and they had penalty shoot-outs for "tied games" and even suggested moving the goalposts (well making them bigger but same principle!) Once the fad of T20 wears out the fools throwing money at the game to make it popular will grow bored, it is doubtful the impact will be quite the same outside of India and the novelty will no doubt wear off. Go play Brian Lara or Cricket 2007 with 20 overs a side and see how long it takes to be bored.
I'm all in favour of minor adjustments, like with cricket games on games consoles there could be some fine tuning to make runscoring more difficult or bowling harder, but you wouldn't make any radical changes.
Owzat added 3 Minutes and 25 Seconds later...
I should add T20 has the "appeal" of being shorter and therefore those that want to watch it on TV or at the ground can work and then go to it with 5.30pm starts. While in theory that is good, not everyone can get from work to a cricket ground in that space of time, the appeal of the opposition has to be assessed and away games aren't in the same county so it could start at 9.30pm and you still wouldn't make it! Plus people do have lives outside of cricket. You can also end with as few as 5 overs a side as rain doesn't stay away just because it's T20, cricket still robs spectators one way or another.