Definately get out. Have a look out to see what is on locally. Can the children have a class (ie swimming/gymnastics/drama) that is on a weekend day? My dd's are 4 and 2 1/2. dc3 due any second (tonight I'm hoping!) My dh often works weekends.
I hate going to the park on my own - so would avoid it if I'm not meeting up with friends. I also hate soft play places.
We have lots of castles where we live so would head off to discover a new one every weekend.
Go to the local markets to pick up veg/fruit for the week
Try and go and do something that you will enjoy too, I do think that is the secret. If you are having a good time too it makes it fun for everyone and time flies.
Walks, collecting nice stones from beach to paint. Driftwood to make 'sculptures'. I like making stuff and get the children to make stuff too. Most of it looks crap but we have a nice time making it.
Go somewhere nature-y and take loads of 'artistic' photos of children. Most of mine look very amaturist but I have a small handful of fantastic shots which are in frames. Both children have very very cheap cameras and love taking photos.
We bake a cake most weekends. It looks a shambles as girls make it from start to finish but they love it and it always get eaten
Some places have toddler groups that are held on the weekend.
Some swimming pools have family time where extra lifeguards are insitu so you can take 2 children into the pool on your own.
Go to ikea for lunch and let them have a play in the free creche if you get desperate!
We will sometimes head off to see family who live away and stay over the saturday night.
Develop your group of mates - make sure that they know that you are on your own over the weekend. I meet up regularly with a friend who's partner goes training on a saturday morning, another who is a golf widow, another who doesn't particularly like her dh so doesn't like to spend too much time with him etc!
Definately plan for a full weekend that takes you up to tea time.