I agree with the other posters that suggest planning your week and meeting up with friends.
I only have one DD (2.8) and I have been at home with her since she was 18 months, sometimes it does get boring (usually when it is raining for days on end).
I always have her registered for a few different community centre classes, crafts, dancing, music etc some which I join in with and as she has got older I have been able to drop her off and enjoy some 'free' time. The classes are usually only 45 mins but it is something to structure the morning around, and gets you out the house and socializing with other mums and DC.
I also like having the next few months planned out, but always leave a couple of free days so not over scheduled.
We also go to the park alot in the afternoon, she is now playing more with other DC, so I am able to relax a bit, we have 3 different parks we drive (5/10 mins away) to and also one in walking distance to keep things fresh. Sometimes we take the bike or a ball to kick around or a kite to fly if it is windy.
In the summer we are able to go to the beach etc which is great. Or play in the garden, which is tiny but we do have a sand/water table, junior swingball, balls etc or we get out the pavement chalk or bubbles.
I am also a annual member of the aquarium so we can go there a fair bit, the library also does free storytimes once a week and there is always swimming etc.
We also have people over for playdates, do baking/crafts with the DCs or just have a coffee and a chat.
Personally I am a lot better a the sociable, activity side than 'playing with toys at home' which I do find boring, I am lucky that my DD has always loved playing with other children and loves to go out and do things too.
You could do all this and still find it really boring though! In that case probably better to go back to work! :)