just been through this so shall tell you what happened with us!
Dd1 was 2.5 when dd2 arrived (now 8mths)
Dd1 went off with DH a couple of days before birth to buy a pressie for baby - but tbh don't really think she understood that much. She went to all ante-natal stuff with me and still thinks hospital is where you get babies and nothing else.
whilst I was in labour DD1 spent day with grandparents - went out to zoo and had a great time and really spoilt. came into hosp next day and was a bit shy but they exhanged pressies and had a cuddle. Oh, I made sure that DD2 was in the cot when they got there not in my arms - I gave DD1 cuddle first and showed her her sister.
once home, life followed normal routine - DD1 was upset with the first bout of screaming but settled afterwards. Nappy changing, feeding etc was a pain - had to do both at same time as DD1 was jealous. She also wouldn't share her changing mat but I had a new one anyway so not a big issue.
we didn't do anything overly special with DD1 except for mummy taking her out on a 1:1 at least once at the weekend.
we kept to same toddler grps etc. But I didn't keep up her childminding sessions or playgroup - these all resumed after 3mths. I didn't want her to think I was "abandoning" her in preference for the baby - also couldn't afford it whilst on mat leave.
It took about 3wks for DD1 to adjust to the situation.
we have more of an issue of jealously now (last 2mths) because DD2 is crawling and attempting to walk and obviously has started picking up Dd1's toys. I really do need eyes in the back of my head now!