Plan a surprise for the Stag: Sweet surprises are always welcome. You can surprise the stag by inviting his long lost friends at the do. You can even decide on the party and venue etc by initially keeping it a secret. After everything is done just casually ask him to drop by at the given place.
Avoid provocative entertainment: Keep the party fun filled however avoid any provocative entertainment that might lead to embarrassing situation for the stag and might even ruin the party.
Keep a check on the boisterous behaviour: it is mostly observed that unrestrained booze often leads to boisterous behaviour from some members of the party that might lead to scuffles and fights. So, it is your responsibility to ensure that enjoyment should not turn into some sort of a nightmare.
Hire professional help: Though it is nice to organise the Swansea stag do on your own but if you want to take the help of event coordinator you can even do that. He can suggest some pretty good ideas that might even surprise you. Hiring an event coordinator can either be your biggest relief or a biggest headache. Hence, choose the professional that would provide assistance to you and not dictate to you. A good event coordinator is capable of providing you with the customised do keeping your needs and preferences in mind.
Well, these were some of the ideas that you can use in making the party a memorable one. However, after zeroing on the idea that you would like to go for you would also need to plan for date, venue, the guest list and the invites.