I am a SAHM and all my friends work either part or full time, so their babies/toddlers go to nursery for at least some of the week. DD doesn't go at all, and I'm always hearing about all the stuff the other kids get up to all day long and feeling guilty that DD is missing out.
I take her to 1-2 x 1-hour groups a week, which involve mostly singing songs and dancing around. The rest of the time is totally unstructured - most days we either have someone round or visit someone (mostly but not always mums with babies) in the morning, then in the afternoon she just messes about, emptying cupboards etc and following me around and chatting while I potter. Or we go out to the shops.
Weekends are probably fairly similar to my friends': DH occasionally takes her swimming, we go to the park, visit friends etc.
BTW DD is very sociable and outgoing. She's behind her friends in walking (which she's not doing yet) and certain skills like eating with a spoon, drinking from a cup - but quite far ahead in terms of speech.
Do I need to find ways of giving DD more stimulation / regular exposure to other kids do you think?