big hugs - we have similar age gaps and both girls so i am trying to remember back
some suggestions based on age and activities i know are still fun for older kids:
1. musical instruments - if you don't have some i highly recmmend starting a collection now - over the years we have amassed a huge collection and even the 6 year old and friends love it. the younger one can do shakers/maraca, tambourine, drum anything... and if you want somehing special for dd1 keyboards are great or a recorder, harmonica, or triangle. my kid put on shows, create a band with friends and march around the house...The best starte kit we had was from ELC - the jungle beats music kit (has drum, maracas, castanets, etc) and the 4 in 1 shaking set that connect all together or can be used in any combination (can't see on their site anymore)
http://www.elc.co.uk/toy/jungle-beats/2. simple crafts/art activities that can also be elevated for dd1:
~stickers/collage work. d2 can scribble and stick on stickers to pages while dd1 can draw and actually do her own cutting and pasting
~playdough - if dd2 is still putting in her muth you can google for recipes for EDIBLE playdough and then not stress. can also add Mr potato head figure bits to extend this, cutters, stampers
~stamps with washable ink pads/paints: you can get big stamps with knob handles for the toddler (we still use ours) and more advnced ones for dd1 if necessary. this is my go to "painting" form when i don't want a mess
~ watercolours - my favourite no staining and easy peasy art (which mine RARELY choose ugh!) or water panting (you knw those magic books with the water painting to eveal the colours?)
3. put on music for them to dance to
4. finger puppets or hand puppets
5. doll play - dd2 is at a good age to start : dolls, clothes to change them, brush their hair, pretend feeding, push them in a doll stroller.
hope that helps. i know it can be frustrating it is to try to keep those 2 age groups happy at the same time...