Gosh, are you me? Also have a toddler, also pregnant with DC 2 and also sometimes bored out of my mind! Doesn't help that we moved abroad when DD was 9 months and I'm in a country where I'm not allowed to work due to visa regs, so no respite there!
I think the thing that helps most is routine, so I try and make sure we have an activity on each day - Mondays, music class, Tuesdays, playgroup etc. I try and make sure these things are in the morning so I have something to get up and out for - I hate the feeling of 'killing time' moping around indoors (granted, it's more difficult in the winter).
The other thing I did was to find a group of other SAHMs and organise that we meet up one afternoon a week eg to go to the park or somewhere indoors if it's colder. I've also learnt to invite others over/for coffee etc even if I don't know them so well - its surprising how many mums were reluctant to do this (me included) but we've all become pretty good friends. (This may be cultural - it's America and there isn't really a SAHM culture.)
I often think back to the halcyon days of maternity leave when I'd be with my NCT group in the local cafe most days but do you know what? That wasn't real life and even if that situation were possible now and most people weren't back at work, the toddlers are not good cafe customers. My DD won't sit still for more than ten minutes - no more huge cuppas and cakes whilst baby sleeps for a couple of hours!
Gosh - this has turned into a bit of an essay! The other thing I think will change is that as DD gets older, she'll need less entertaining and be more able to play on her own. At the moment she is into everything but wants me there with her to build towers etc and it feels like I never get a moment to myself. Finally, once the new baby is here, you'll wonder why you didn't make more of all the free time!
The grass is always greener too, remember. I've got friends who would give anything to be a SAHM!