Question for the experts on the forum from a newbie. My current plan stands at:
1. reaching Frankfurt on Apr 21(Fri) @ 10:30 am from LAX.
2. FRA-PARIS (need to decide travel mode) immediately.
3. spending weekend in Paris until 6 pm on April 23(Sunday)
4. PARIS-AACHEN April 23 night in Aachen and entire week in Aachen for conference.
5. AACHEN-FRANKFURT April 29 for flight back.

Questions:
FRA-PAR: Flights are too expensive and so I need to take the train or rent a car. Renting car (Merc C Class) works out to 200 Euro from Apr 21-Apr 24(pickup at FRA, drop off at Aachen). Alternatively there's a train leaving from FRA Main @ 12 pm reaching Paris at 5 pm. I'll need to take a train from Paris to Aachen (which is 3 hours).

a) Is it a nice drive from FRA-PAR, and worth the hassle? I love driving, but if its too big a pain might just take the train.
b) If I take the rental, what's the option to park in Paris, do hotels have parking nearby or I'll have to find parking in public lots?
c) If I reach FRA at 10:30 am, is 12 pm from FRA Main a possibility or do I need more time?
d) What happens if I miss the 12 pm train, can I get on the next one?
e) So if I decide to take trains, I might take the Eurail (3 country 5 day pass for $383). Any better option?
f) After conference on Friday, I have Friday evening and a bit of time on Saturday morning, is it worth spending time in Aachen or heading out Friday evening to Frankfurt and spending the night in FRA?

Would really appreciate any help or directions on which websites to look at for trains/details etc!

I'd just take the train. Should be under 90 euros one way FRA - Paris. I'd think the whole trip should be cheaper by train once you figure in gas and parking fees, etc.

If you've been in Aachen for a week already, won't you have seen a few things? It's unclear what time your flight departs on 4/29, but on Fri you might want to just catch a train out of town and spend the evening somewhere near the airport, like Mainz, a pleasant place to stroll, shop, have a meal, and only about 20 minutes from FRA by S-Bahn or regional train. Try to stay near the old town, near the Dom, for the best atmosphere.

I would recommend a train.
First, though, you don't need to go to Frankfurt Main to catch the train, most trains stop at the airport (Flughafen). Which gives you more time, as the train arrives at the airport at 12:13, instead of noon. You change to the Thalys in Brussels; an easy change as they will arrive on tracks close to each other. You only have 8 minutes between trains, but even if your ICE to Brussels is late (it happens) and you miss the Thalys, they run frequently and you will be able to catch the next one (without charge; if you miss the train because your connecting train is late, they'll rebook you--it has happened to me more than once).
Re missing the noon train, wait and take the one leaving an hour later, as the other trains leaving between noon and one pm require more than one change.
Don't drive...you'll be jet lagged, it's a long drive and traffic around Paris would be pretty heavy. You will do far better to relax on the train, enjoying a glass of the beer of the month in the dining car on the ICE train. Trying to drive that much the day you arrive will be exhausting.
DO NOT buy the railpass. The ticket for Frankfurt to Paris will cost you somewhere around 100 euros and ditto for the train to Aachen. Point to point tickets will be about the same price or less if you buy in advance.

Re Aachen, I hope you are staying at the Quellenhof (now a Dorint/Sofitel). It's the nicest hotel in town, handsome setting, easy walk to the old town (we usually park in the garage next to the Quellenhof). If not, you might be staying at the Best Western. Not as nice a hotel, but a closer to the old town and has a decent Japanese restaurant. In the old town itself, the Rose am Dom is a pleasant traditional restaurant that dates back to the 1300s.
On your last night, you could go to FRA. But how early is your flight the next day? If it's not too early, you could either go to Cologne or Dusseldorf. Dusseldorf is a sophisticated city with a lively old town. Highspeed ICE trains can whisk you to Frankfurt airport from Dusseldorf in 1.5 hours, no changes, and tickets cost 63 euros.
Hope this gives you some ideas.

thanks for the quick response guys! I completely forgot the jetlag thing...totally makes sense to take the train with a drink or two. And helps that you dont get screwed for missing the train.

My flight out of FRA is at 12 noon on 4/29, so not much time in the morning, might have a bit in the evening on Friday. I will be leaving Aachen @ 2 pm, and so should get to FRA by 5-6 pm.

As for Aachen, the conference is in Netherlands, a few miles from Aachen @ http://kasteelvaalsbroek.dolce.com/. No idea if its a good place, looks decent from the pic. Anything nice around the Aachen area for the evenings? thanks agains the reply!!

or alternatively, I could get to Dusseldorf for the evening on 4/29, spend the night there and take a morning train to FRA. Is that a better idea?

The hotel looks great, should be a lovely time. I hope you still get to spend some free time in Aachen or the surrounding area to see a bit.
My vote is still with the Dusseldorf option--but we simply like the city. The ICE trains would get you to FRA airport in plenty of time.