Rent lots of movies - especially the ones that only you want to see!
I would also suggest universalclass.com where you can actually teach & earn money (would be stimulating mentally) (or even learn yourself).
Do a pregnancy blog/online baby book and/or start a scrap book - I'm sure you're little 3-yr old would love to help with a scrap book for the new babies (and would be a great way of getting her involved and prepared for what's to come).
Read up on your books & refresh your labor/birth knowledge so you can be prepared.
I'm sure you could find a wheelchair to rent or buy a cheap used one online to get around when you need to move about and get out of the house - take a family stroll in the evenings, or go to the mall/get outdoors/go to the park.
Check out this yahoo group:
On the thank-you side of the question... Give him a night out w/ his friends - prepare yourself w/ take-out, and movie...etc so you will have everything you need (with a relative/friend at a phone-calls notice). Or, call up some friends and play some board games/card games so you don't have to be up and about. Give a backrub in bed (relaxation is important for childbirth prep anyway, so, this is a perfect time to do your relaxation exercises).
Hmm... I think I had more suggestions for the bedrest part! :)
Main thing - try to think outside of the "i'm stuck in bed" box, and think of all the things you can do without being active - it really depends on the things he likes to do and what is available in your area.
On the "childbirth educator" suggestion side - stay low stress, do some yoga/meditation type stuff - you seem rather happy, but, you seem like you could be a little stressed that your husband is doing so much. Don't worry - this is the time where he should be doing a lot - you're growing two babies, he can handle housework.
Get lots of protein and 64-96oz of water each day. Pee a lot!!! Get your fiber!
Brush up on your breastfeeding knowledge & work out a birth plan! If you haven't already - read Ina May's Guide to Childbirth and Creating your Birth Plan by Marsden Wagner