what about things like a simple walk to the park / feed the ducks / trip to local farm / country park for walks etc
Do you know many other mums.... just getting together at each otehrs houses for a coffee and chat while the children play is often more than enough, especially when they are so young.
Do most of your friends have travel cots / booster seats as spare highchairs as if so you may be able to find that the children can play, then have lunch and then a nap and then you can have 2 hours of quality adult time.
I know what you mean how being in the house can just be too tiring to contemplate, at 8 in the morning when you have no plans and think WHAT will i do for the next 11 hours!, but just getting out of the house for a short walk can often be more than enough.
Especially right now they are SO young.
Im nowhere near you but all my localy lesiure centres have a coupe, of sessions per week where they fill the sports hall with A bouncy castle, tiny tramplonies / mats / balls / mini ball pool/ little tykes cars etc where you 15month old could tire herself out and baby could easily watch on the side in his car seat and be safe, or is you really wanted be carried around in a sling.
what about music groups, again there is one particular one where i live that is fantastic and much better than the rest... its hust a question of trying them out and word of mouth from other mums!
None of the above suggestions would cost anything for the tiny baby, and would not need more than 1 adult and cost no more than a couple of pounds max!
I would agree swimming would be tricky but all the pools round here allow 1 adult to 2 unnder 8s, and 1 pool allows 1:3 (wouldnt do swimming with a 3month old AND 16 month old though, at least not for a few months yet!)