I always enjoy organising parties for my four children. Parties for 4 - 6 year olds are, for me, the perfect age, as their expectations aren't too high. We book a venue (usually a village hall or similar) that isn't too expensive but allows plenty of room for children to run about and have space. Would normally book the party either a few days before or a few days after the actual birthday and presents are never opened at the party (too much else going on, pieces get lost, lose track of who bought what etc).
There is usually a theme for the party but isn't too rigid - for example, my daughter had a Disney Princess theme for her 4th Birthday party and dressed as Belle. The plates, cups, napkins and tablecloths all followed the theme and the cake and mini cupcakes also matched, but then I bought Monsters Inc plates etc and made some Monsters Inc cupcakes for the boys that were coming along.
Two hours is plenty long enough for a party - normally 1 hour play, lunch, another 30 minutes play, cake and then free play until party end. Traditional games always work well - pass the parcel, musical chairs, a version of pin the tail on the donkey (think pin the tiara on the princess). If there are more than 15 children at a party, I have two presents going around in pass the parcel to prevent it going on for too long.
In terms of food, I keep it simple. I always provide some sandwiches but only allow two quarters per child maximum - usually a mixture of jam, ham, marmite and cheese spread. Cocktail sausages, sausage rolls, breadsticks, cucumber and carrot sticks, cherry tomatoes and crisps to start, followed by cake and biscuits (party rings & chocolate fingers) - to prevent waste, I make the mini cakes (about 1" in diameter) with icing and decoration, as many children are more interested in what is on top. Drinks are usually jugs of water or weak squash. I always buy a stack of cheap plastic cups as extras as children get thirsty and need endless cups of drink and not always possible to keep track of cups. I also take along cakes, tea and coffee so that there is something for any parents who stay.
The best part of having the party at a venue is that when the party finishes, you can throw everything into black sacks or food storage containers, load the car up, drive home and have a well deserved cup of tea whilst the birthday child gets to open their presents - only job you need to do is record which guest bought which gift, ready for the thank-you letters!