The lack of a committee letter doesn't seem to be much of a problem for most schools. I know that several prefer that you send one, but so long as you have the required letters (biochem/chem/bio/dentist/etc) for each school you apply for, they don't seem to mind all that much. I didn't have a committee letter and two of the professors I asked for LORs barely knew me, but I still was able to interview at several places, so I'm not entirely sure that LORs really make much of a difference in an application unless they say something particularly good/bad about you.
Regarding your second concern, I'd definitely recommend finding something to put down as an extracurricular. Any GPA over 3.5 is great, so you're good on that front, but you want to show that you're an active person outside of school as well. Rather than trying to find extracurriculars that will look "good" on an application, find something that you are personally interested in and get involved with that. It'll be more fun and it'll actually look better on an application, as it'll tell the admissions committee something about who you are and make interviews more interesting for both you and the interviewer. Almost everyone applying to dental school will be involved in the Pre-Dent Club or the Health Careers Club, but very few will be members of the Breakdance Club or the Underwater Basket Weaving Club.
One last point of note - make sure to shadow with a general dentist as well. It's fantastic that you were able to look at specializations, but you're applying to be a general dentist first, so that's what they're going to be talking to you about. 🙂 Best of luck applying and good luck on the DAT!