I really sympathise. My 2 are 2 and a half years apart. It's terribly hard being pg with a toddler as well.
I wouldn't withhold park visits or other treats at this age, I think that is just shooting yourself in the foot. Fair enough, if she is naughty while you are at the park, warn her you will leave, and then leave if she carries on (or if it is particularly bad, leave straight away).
I know it is really hard, but if she does things like hitting you, try to remain calm and firm, keep your voice low, look her in the eyes and say "NO hitting". This is going to sound a bit weird, but try not to take it personally - she is too little to mean to hurt you, it just feels that way to you.
Maybe she senses things are changing and feels confused. Lashing out is the way she expresses this. If you cry, it will make her feel more vulnerable, so please try not to do it. If you feel very emotional, leave the room (I know what it's like, I struggled with depression while Pg with DS2)
Does she go to playgroup ? - if not, I'd really suggest that she does - to give you time apart, and opportunities for you to miss her , and as a routine to go back to once the baby is born.
She is still your lovely little girl and this is just a phase which will pass - try not to jump ahead and worry about the future (if that is what you are doing).
Look for any good behaviour and praise her to the skies. I used to give DS2 stickers to put on his coat, for instance, if he behaved well in the shops.